Laws and Customs Regarding Making a Siyum During The Nine Days

Making a Siyum During The Nine Days

The Shulchan Aruch (551:10) rules that one may not eat meat or drink wine during the nine days unless it's a Seudas Mitzvah. Included in the list of what's considered a Seudas Mitzvah is making a siyum. 

What is the source that a siyum is a Seudas Mitzvah? Can one make a siyum during the nine days even if he finished his learning earlier, and eat meat when it's typically forbidden, by saving a part of the tractate for the siyum during the nine days?

The Shulchan Aruch in Yoreh Deah (246:26) rules that when one finishes a tractate, it is a mitzvah to rejoice and make a meal, and that it's called a Seudas Mitzvah. This is based on Gemarah Shabbos (118b), which states: "Abaye deserves a reward, because when he sees someone complete a tractate, he makes a feast." 

Many Acharonim, including the Elya Rabbah (551:26), Aruch HaShulchan (551:28), Chai Adam (133:16), and Mishna Berura (551:73), rule that one should not plan to complete his learning during the nine days. He should not speed up or leave out a part of the meat during this time.

However, Kaf Hachaim (551:161) presents a proof from Moed Katan (9a) that one can postpone the completion until a suitable time (the Aruch HaShulchan argues against the evidence). 

In practice, the custom is always to make siyumim during this time, whether one finishes it at their regular pace, slows it down, or quickens their learning. (Kovetz halachos 9:37).

Notes: 
Many Chasidim offer two reasons why we should make siyumim during this time: it encourages extra learning, and it fosters greater achdus when everyone comes together for the siyum.