For some reason there was a period of time when John had to spend a while in New York - he moved himself and Dean and Sam into a predominately Jewish neighbourhood. Although Dean never really liked the big city, he acclimatized to his little neighbourhood and before long he was able to fit right in (as he is always able to do). He was just another one of the boys. He possibly also attended hebrew school along with his friends - either because John thought it was a good learning opportunity, or because Dean just wandered in one day and no one kicked him out.
The reason this is my fanon is because of Dean's tendency to use Jewish phrases. Out of the two brothers, it's Dean, not Sam, who uses them the most. Why New York: Dean's phone number for a good long time started with a New York area-code - indicating to me that he might have been in New York in the mid-to-late 90s, when personal cell phones started to become more common...and he liked his time there enough that he kept the number for nostaligic purposes.
It's likely that Sam was there too, but Sam, as always, had a harder time fitting in and making friends. He was envious of Dean and didn't like New York at all - or wouldn't have, except for the museums and library. Sam spent most of his time alone, or tagging along with Dean and his friends. These days though, Sam looks back on those days fondly. Dean, of course, has long since lost touch with all his friends. He never said goodbye - he just slipped out in the middle of the night as always and that was that.