It is a mitzva to act with reverence in a synagogue, for the Torah says: “Revere My temple” (Vayikra 19:30), and the sanctity of the synagogue is a reflection of the sanctity of the Temple.
Whoever uses the synagogue for non-holy everyday purposes dishonors its holiness, since the synagogue was designated solely for sacred matters: Torah, prayer, and strengthening other mitzvot. Therefore, one should not discuss business and finances there, and certainly one should not joke or gossip there. However, it is permitted to hold assemblies for the sake of a mitzva, such as to collect charity for the poor or to raise funds for yeshivot.
It is a mitzva to honor the synagogue and maintain its cleanliness at all times. One who has mud on his shoes must remove it before entering the synagogue (SA 151:6-9).
One must not enter a synagogue unless it is for Torah or prayer. Even when it is raining outside, one may not take shelter in a synagogue. Likewise, one may not enter a synagogue in the summer to take refuge from the heat. However, if one intends to learn or pray in the synagogue while taking shelter, she may enter.
One who must enter the synagogue to get his friend’s attention regarding an urgent matter may do so; however, before doing so, he should sit a bit and recite a verse, so as not to enter solely for personal needs (Megilla 28a; SA 151:1).
In a synagogue that does not normally accommodate se’udot mitzva (festive meals celebrating the fulfillment of a mitzva), it is preferable le-khatĥila to follow the stringent opinion of Magen Avraham and not hold large se’udot mitzva at which wine is served, like those of a brit mila, pidyon ha-ben, bar-mitzva, or sheva berakhot. However, serving light refreshments there at the celebration of a joyous occasion, such as a siyum (celebration of the completion of a talmudic tractate) and the like, is permissible. If it was customary to hold se’udot mitzva in the synagogue from its inception, then all agree that it is permissible to do so. This, of course, is all on condition that the congregants are extremely careful to preserve the sanctity of the synagogue, that they refrain from all idle talk and frivolity, and that they make certain not to become intoxicated. They must also ensure that women and men do not mingle in the synagogue. Those who wish to follow the lenient position may conduct se’udot mitzva even in a synagogue that did not initially host such events, for that is the opinion of many poskim (see Peninei Halakha: Collected Essays I, 6:8; for further study, read that entire chapter).