Echad Vachamishim Umatayim - mi yodeya?

Question

Who knows two hundred fifty-one?

Please cite/link your sources, if possible. At some point at least twenty-four hours from now, I will:

• Upvote all interesting and relevant answers.

• Accept the what I think is the "best" answer.

• Go on to the next number.

Back to hard ones, a prime in this case.

Answer 1

There were 251 years from Avraham's birth to Yosef's.

Answer 2

The coming Shavuos (first day outside Eretz Yisrael, only day in it) marks 251 years since the passing of the Baal Shem Tov (in the year 5520/1760).

Answer 3

From 1805 to 1855 (which are apparently the years for which records are available), the London bes din converted 251 minors who had Jewish fathers. (Source.)