When one completes a single Tractate of the Talmud he makes a siyum which is celebrated with a festive meal which the Code of Jewish Law defines as a “mitzvah meal” (Shulchan Aruch, Yoreh Deah 246:26). The source for this practice is the Sage Abaye (Tractate Shabbat 119a) who declared that when he saw one of his disciples complete a tractate he made a festive meal for all of his students.
One reason for this celebration, says the Gaon of Vilna, is the explanation given by the Sages (Tractate Bava Batra 121b) for the Fifteenth of Av being such a special day of joy. It was on that date that the cutting of wood for use on the altar of the Temple was completed for the year. Reaching the climax of a mitzvah is a cause for joy, and so is reaching the climax of completing a course of Torah study.
Maharsha, however, suggests another source. The Midrash states that we learn from the feast which King Solomon made for his entire court upon being blessed with the extraordinary wisdom he had requested in his nocturnal dialogue with God (Kings I 3:15), that we should also make a feast when completing the study of the Torah. This is the source for our celebration on Simchat Torah when we complete the reading of the entire Torah, and for our celebration of the siyum of a tractate. Just as Solomon understood that there could be no greater cause for joy than the gift of wisdom, likewise we celebrate the acquisition of wisdom which comes from a year's reading of weekly portions of the Torah and also the wisdom that results from completing a tractate.
To you and to all participating in this great celebration I wish a hearty "Mazal Tov"!
