http://www.teach613.org/index.php?option=com_content&task=view&id=162&Itemid=54When to make a Siyum: What qualifies for a Torah celebrationSources
1. Abaye said: If I saw a scholar who completed a tractate, then I would make a holiday for the Rabbis. (Talmud, Shabbos 118b-119a)
2. When a person completes a tractate (mesechta) there is a mitzvah to rejoice and to have a festive meal which is called a mitzvah meal. (Ramoh, YD 246:26)
3. The final day is called Simchas Torah, because we rejoice and make a festive meal for the completion of the Torah. (Ramoh OCh 669:1)
4. The fifteenth day of Av was a day of celebration because it was the day on which they completed cutting wood for the Mizbeiach. (Talmud, Baba Basra 121b)
5. "Shlomo awoke, and behold it was a [valid] dream. He came to Yerusholayim and stood before the Aron, and he brought sacrifices. He made a feast for all of his servants." From here we learn that one should make a festive meal for the completion of [a] Torah [endeavor]. (Medrash Rabboh, Shir HaShirim 1: 9; Melochim 3:15)
6. The completion of in depth study of a volume of Torah can be considered a "mitzvah meal". We find that any mitzvah that extends for a long period until its completion has justifiable joy at its completion.
(Igros Moshe OCh 1:157)
Applications
7. There are those who do not eat meat or drink wine during the week in which Tisha B'av will occur. Some add on the time period from Rosh Chodesh Av. At a "mitzvah meal" such as a Siyum on a tractate, anyone who is included in the celebration may eat. During the week of Tisha B'av only ten people are allowed. (Shulchan Aruch 551:10)
8. A firstborn fasts on the day before Pesach, to remember the miracle by which the Jewish firstborn did not die during the tenth plague. The custom, however is, that they join in a siyum and eat. Even though they did not study the tractate, they may join the joyous occasion of the person who did finish the study. (Shulchan Aruch 470 and Mishnah Berura 1 and 10)
9. The statement "If I saw a scholar who completed a tractate, then I would make a holiday for the Rabbis" indicates that the celebration included those who were part of the learning group even if they didn't complete. There are those who allow a firstborn to eat if they study a short volume of Torah with Rashi's commentary. (Teshuvos V'Hanhagos 1:300)
10. If a person would be effected by a fast such that he would have trouble doing the mitzvos of the seder properly, then it is better that he shouldn't fast. (Mishnah Berura 470:2)