Wikipedia
A siyum (“completion”) is the completion of any unit of Torah study, or book of the Mishnah or Talmud in Judaism. A siyum is usually followed by a celebratory meal, or seudat mitzvah, a meal in honor of a mitzvah, or commandment. Siyum also refers to the celebration.
An enduring custom is for the community to complete a unit of Torah or tractate(s) of Talmud during the 30 days following the death of a beloved one and hold a communal siyum thereafter, in tribute and honor of the memory of the deceased.
It has become customary for synagogues to arrange a siyum on the morning before Passover to allow those fasting for Ta’anit Bechorim ( Fast of the Firstborn) to break their fast, taking advantage of the halakhic principle that prioritizes Torah study.
A siyum ha-sefer, meaning “completion of the book,” is also held as a ceremonial completion and dedication of a sefer Torah, a handwritten copy of the Torah, the most important Jewish ritual object, which is kept in the Ark of a synagogue.
Usually, when an individual or a group conclude the study of any tractate of the Talmud, or even of the Mishnah, a siyum is celebrated. At the end of every volume of the Talmud a special hadran prayer is printed with a set order of prayers and a special kaddish, Kaddish D'itchadita, in honor of the completion of that volume, which Judaism considers to be an important achievement and a milestone worth celebrating.
In the merit and honor of a deceased individual, it is customary to undertake Mishnah study with the goal of holding a siyum.
Rabbi Moshe Feinstein, based on the Nemukei Yosef, the Ran ( Rabbeinu Nissim), the Rashbam, and the Eliyah Rabbah, extends the concept of a siyum to include even a festive meal celebrating the completion of any mitzvah (commandment) that has taken a significant duration of time (such as a number of weeks or months).