Donald Trump Hits NYC Bar Mitzvah -- You're My Jewprentice

There comes a time in every boy's life ... when Donald Trump shows up at your Bar Mitzvah -- at least that's what happened to one kid in New York last weekend.

The photos were snapped Saturday night ... at a Bar Mitzvah for the son of Trump's lawyer -- we won't make a joke here, it's too easy -- and Trump had a blast.

We're told Trump hung around and schmoozed for a while -- and even gave a speech at the coming of age celebration. One source at the event tell us, "It was very heartfelt ... everybody loved it ... and he took tons of photos with everyone who asked."

And here's a shocker -- in typical rich-kid-Bar-Mitzvah fashion, a famous person performed at the party ... in this case, "Down" singer Jay Sean.