Officially, the Mostly Mozart Festival begins tomorrow night, with a gala concert featuring the “Coronation” Mass. (By Mozart, I should perhaps say.) But Friday night, music director Louis Langrée and the Festival Orchestra gave what they called a “preview concert.” It was an hour and 15 minutes — no intermission — and free to the public. And the public came, in very high spirits.

They dressed casually, they applauded heartily between movements, they let babies cry (to the extent they had a choice). And Mr. Langrée was fairly relaxed too. He wore what has become almost standard in the concert world: that Mao shirt, as I call it — only this one white, for summer.

The program began with a novelty: the Toy Symphony by Leopold Mozart.This piece is also called, in charming German, the “Kinder-sinfonie.” Leopold, of course, was Mozart’s father — Wolfi’s father — and a fine musician in his own right.You might even think of him as an important one, given the book of violin instruction he penned.

Among the instruments for the Toy Symphony are…well, toys: a rattle, birdcalls, and the like. Many of these were played by children of the orchestra members, who, naturally, looked adorable. And a regular member of the orchestra played a bright, dinky little trumpet (to use technical language).

The Toy Symphony is a pleasant piece in C major, although it grows just slightly tiresome. Mr. Langrée and his forces performed it admirably. Even in this little novelty, we heard Mr. Langrée’s hallmarks: crispness, gracefulness, and definition.

The program continued with a piece by Wolfi himself: the Symphony No. 1 in E flat, K. 16. That’s an awfully low Köchel number, isn’t it? Mozart wrote this piece when he was 8. And, in his program notes for the evening, Paul Schiavo told a story about it.

According to Sister Nannerl, Leopold fell ill in the summer of ’64,and she and Wolfi were not allowed to make any noise in the house. So the boy occupied himself by writing a piece of music, most probably this symphony. He wrote it directly onto the page, without benefit of the keyboard.

You know who else does this? One of our most interesting composers, Jay Greenberg, who is now a senior citizen of 14. The Sony label has just released his Symphony No. 5 and his Quintet for Strings. They, and their creator, are worth paying attention to.

In any case, Mozart’s Symphony No. 1 is an excellent piece — a simply wonderful piece. It’s not just amazing for an 8-year-old, it is an outstanding piece of music, by anyone, at any age. The symphony is smartly crafted, as you might expect, but also really beautiful.The slow movement is a haunting thing in C minor, utterly typical of this composer, all life through. (Short as that life proved.)

From Mr. Langrée, we again heard those hallmarks.And to elaborate just a bit: He knows how to judge space between notes; he is a keen observer of rests. A piece, in his hands, tends to be alert and alive.

The concert closed with Mozart’s last symphony — No. 41 in C major, K. 551, called the “Jupiter.”Thus was symmetry (or something) achieved. The “Jupiter,” needless to say, is not only one of Mozart’s best pieces, but one of the best pieces in music— and, when you get down to it, one of the greatest works of art.

Mr. Langrée and the orchestra played it commendably. The first movement was very bouncy, in that established “period” fashion. One might have wished for a touch more grandeur. The musicians sometimes sounded like insects, straining mightily to be loud and important. And I might say that the orchestra played better collectively than the members did individually — which is a tribute to the conductor.

I might say, too, about Mr. Langrée, that he is a sniffer. That is, he inhales loudly through his nose, when he wants the orchestra to come in big after a rest. (And this happens a lot in the “Jupiter” Symphony.) This loud sniffing is perfectly understandable, but is better avoided, if at all possible (and it is).

The second movement, Andante cantabile, was not too slow, blessedly. It had its quietly sweeping power. Like the Allegretto of Beethoven’s Seventh Symphony, this music depends almost entirely on pacing — and Mr. Langrée was equal to the task.The third movement — the minuet — had the right combination of relaxation and tension, and the Finale was its glorious self. Mr. Langrée made the music super-clear, and I actually heard things in it I hadn’t quite noticed before, in 1,005 listenings.

At one point in the Finale, a cell phone went off (inevitably). Mr. Langrée, conducting all the while, turned around and gave the audience a look that said, “Oh, criminy.”

I felt sure that he and the orchestra would play an encore, probably the overture to “The Marriage of Figaro.” The audience was certainly enthusiastic. But Mr. Langrée called it a night, perhaps thinking that the “Jupiter” couldn’t be topped, or that an hour and 15 minutes was enough.

***

The day after this Mostly Mozart “preview,” Mannes College’s International Keyboard Institute and Festival saw its last recital. Its last professional recital, that is. There were student performances yet to come.

The Mannes festival had opened on July 16, with a recital by its founder and director, Jerome Rose. Mr. Rose then presented a slew of colleagues much in his mode: Romantic, heartfelt, bold. There is an element of throwback in this festival, and that is to the good. One can hear tidy, cautious, buttoned-down pianists any old time.

Giving the last recital, on Saturday night, was Marc-André Hamelin. Mr. Rose’s festival favors big technique, and, boy, does this Montrealer have a lot. In fact he and Leslie Howard — who played on July 21 — own about 60% of all the technique that exists in the world.

Mr. Hamelin is known for playing hidden, offbeat repertoire that is very hard — and he gave us some of that on Saturday night. This came in the form of Paul Dukas’s piano sonata. (What, you hadn’t known that the composer of “The Sorcerer’s Apprentice” wrote a piano sonata? He did.) But the pianist also played a workright at the heart of the canon: Schubert’s sublime Sonata in B flat, Op. posth.

The Dukas sonata is not a neglected masterpiece, I’m sorry to say; it is merely a neglected piece. Dukas’s opera “Ariane et Barbe-Bleu,”staged at City Opera last season, is a better piece. But the sonata, like the opera, is worth hearing. Both were written by a man who had obviously experienced “Tristan und Isolde” once or twice. The sonata is sprawling, fantasy-filled, and long. The second movement is probably the best of the four. A kind of slow movement, but not really, it contains some lovely ideas.

Thanks to his big technique, Mr. Hamelin was able to manage Dukas’s waves of notes.It’s amazing how relaxed this pianist is as he plays. Actually, it’s not so amazing, because, without that relaxedness, he couldn’t play all those notes. Mr. Hamelin displayed some nifty pedaling, and some nice soft playing. Perhaps best of all, he made this wild sonata cohere — as cohere it can.

And how did the Schubert go, after intermission? Pretty well — although the first movement was problematic. Mr. Hamelin misjudged the very first note, playing it too loudly, giving it a wrong accent. He didn’t do it when the same phrase returned. In general, he played this first movement very, very freely, abusing his privileges as an interpreter. The music would have benefited from a much steadier tempo. Also, Mr. Hamelin’s dynamics were strange, and those dynamics included fortissimos out of proportion to the rest of the notes.

I have said that Mr. Hamelin made the Dukas sonata cohere; he did something like the opposite to Schubert’s first movement, stretching things unkindly.

But the second movement, the slow movement, one of the glories of all the piano literature? Beautifully judged, transfixing.That was worth the price of admission all by itself.And the Scherzo was snappy and graceful, just as it should be — although Mr. Hamelin had a problem that afflicts lesser mortals: Some of the notes failed to sound.

I spoke of the first note of the first movement — how about the first note of the last movement? It’s a funny one, an unexpected, sustained G (in a B-flat piece).Pianists tend not to know how to deal with it. How loudly do you play it, and how much of an accent do you give it? Mr. Hamelin knew — he played the note perfectly (in all of its iterations).

He did well with the rest of the movement, too, although some overpedaling resulted in some unwanted blurring. He also messed up the final passage in the right hand — which hardly mattered, especially in the context of so spiritually great a piece. I mention it only because Mr. Hamelin is such a wizard.

The Mannes College audience gave him a rhapsodic ovation. But he gave them no encore. Back and back he was called, and he simply bowed, sometimes looking a little apologetic. Normally, performers play an encore — or four — whether you ask them to or not. They won’t let you make for the exits. But Mr. Hamelin was asked, over and over, and he declined. Apparently, he did not want to put a piece on top of the Schubert sonata.

A good call.