Q. Is there a source for making a special seuda at the fifty year yortzait?

A. It is difficult to find a source for this minhag. In the quarterly journal Tzefunos (5752, Vol. 18) there is a letter from the Zichron Shmuel answering a query from a talmid of the Chasam Sofer on making a special seuda on the fiftieth yortzait of the Chasam Sofer. He writes that although there is mention of arranging a day of learning on the day of yortzait of a great person (See first Rashi, Yebamos 122a) by his kever, or the seudos of Lag Baomer in honour of Rav Shimon Bar Yochai, (and other tzadikim and parents every year), there is no mention to do so especially on the fiftieth anniversary. He debates whether there may be an issue of chukos hagoim involved (see prior question). Nitei Gavriel ( Avelus 2: 77: 22 n. 40) mentions that the chasidim of Gur do not make a seuda on the day of the yortzait of their Rebbes after fifty years, with the sole exception of the S’fas Emes.
As mentioned in the prior question there are reasons for tikun (mending and correction) in a fifty year period. (See also Ramban Shemos 21: 6)
I enquired a number of Rabbonim in the city if they are aware of such a minhag, to no avail.
Horav Shlomo Miller’s Shlit’a opinion is similar.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a