Question:
If one learned a masechta but never made a siyum, is it OK to make a siyum even weeks or months later, for the sake of erev Pesach (so that the firstborn can eat)? Or is there a time limit?
Answer:
One should only make a siyum if he finished the mesechet etc. on erev pesach or during the nine days. ONe may learn a little bit slower in order that it comes out that he finishes at that time, but if he learned the mesechet already then it is improper to do.
Sources:
Shut Mekor Chaim ( R’ Segelovitz) 36, R’ Shlomo Kluger in sefer Hachaim 551, Eliyahu Rabba 551-26, M:B 551-73, there are those who argue but see Yichave Daat 1-40, and Toras Hamoadim Bein Hamitazrim 5- 52.
