It is customary for all firstborn males to fast on Erev Pesach in order to recall the tenth and final plague which God inflicted upon Egypt – the death of the firstborn. Nevertheless, the Fast of the Firstborn is actually a fast which rarely takes place. This is because it has become universal custom to exempt oneself from the fast by attending a seudat mitzva, a meal celebrating the performance of a mitzva. Most commonly, a meal accompanying a siyum meseches.
There is a great discussion amongst the poskim as to which areas of Torah study qualify for a siyum upon their completion. The most widespread practice is to reserve the siyum celebration for the completion of a tractate of Gemara.
The Pnei Yehoshua (Brachos 17a) deduces that Rav Yochanan would make a seudas siyum when he would finish Sefer Iyuv. Similarly, Harav Meir Arik zt”l (Minchas Pitim Y.D. 246:26) writes that the meal accompanying a siyum on one of the 24 seforim of Tanach is considered a seudas mitzvah.Harav Moshe Feinstein zt”l (Igros Moshe 157) likewise rules that one may make a siyum on a sefer on Tanach, however, only if one learns it in depth with a commentary of the Rishonim (such as Radak etc.) and not with modern day commentaries. Harav Shlomo Kluger zt”l is of the opinion that a siyum on a sefer of Tanach is only considered a halachic siyum if one learned it and happened to finish by Erev Pesach. However, to learn in order to finish and make a siyum on Erev Pesach would only suffice if one is doing so with a mesechta of Gemara and not with a Sefer of Tanach.
As stated earlier, all agree that one may make a siyum on a tractate of Gemara. There is an interesting teaching of Rav Yosef Mashash zt”l in the Sefer Mayim Chaim (179) who writes that one can actually make a siyum on a chapter of Gemara. However, there is room to question this opinion. As the Pirush Anaf Yosef (Medrash Shir Hashirim 1:9) writes that one can only make a siyum upon completing an entire section of Torah and that as long as the section is incomplete it is not considered a seudas mitzvah to permit bechorim to eat.. According to the Anaf Yosef it is hard to beleive that upon completing merely a chapter of a mesechta one can make a siyum and permit bechorim to eat. Indeed, the majority of poskim, including Harav Ovadia Yosef zt”l (Yabia Omer 1:27:10), Harav Gedalya Felder zt”l (Yesodei Yeshurun vol. 6 page 44), Harav Yitzchak Weiss zt”l (Minchas Yitzchak 2:93) and Harav Betzalal Stern zt”l (Btzeil Hachachma 2:28), maintain that a siyum on one perek would not suffice to allow bechorim to eat on Erev Pesach.
The Maharsham (Daas Torah 551:10) cites a view that one may make a siyum on one mesechta of Mishnayos. A similar view is attributed to Rav Yizrael of Rozin zt”l. (See Orchos Chaim 551:35) Harav Yitzchak Eisik Liebes zt”l (Beis Avi 2:52) writes that in theory one should be able to make a siyum on one mesechta of Mishnayos, however, he has never seen such a thing done practically. Indeed Harav Betzalel Stern zt”l (Btzeil Hachachma 4:99:2) writes that while learning a mesechta of Mishnayos is a big mitzvah, one cannot make a siyum on it and permit bechorim to eat on Erev Pesach. Harav Azriel Hildsheimer zt”l (Shu”t Rav Azriel Y.D. 246) and Harav Shlomo Zalman Auerbach zt”l (Halichos Shlomo Pesach page 181) also feel that one cannot make a siyum on one Mesechta Mishnayos.
The Shu”t Pnei Meivin (103) writes that while one mesechta of Mishnayos would not suffice for a siyum, one seder of Mishnayos (such as Zeraim or Moed etc.) is considered a siyum. This is also the view of Harav Betzalel Stern zt”l. Harav Yaakov Kamanetzsky zt”l (Emes LYaakov page 225) also rules that one can make a siyum on a seder of Mishnayos.