By a Seudas Mitzvah event [i.e. Bar Mitzvah, Bris, Pidyon Haben, etc]:

Bar Mitzvah: This follows the same law as a wedding meal [due to the dancing and great extravagance with which it is organized today] and hence a Mechitza is required. Nonetheless, some Poskim[1] conclude that it remains permitted to participate in a Bar Mitzvah celebration without a Mechitza which follows the lenient approach so long as the dancing is separate and the women are dressed modestly and some require also that there not be mixed seating.

Bris:^[2]  Women should not be together with the men, and the men should not be together with the women, by a Bris, as this is considered Peritzus. [However, many are accustomed to not be particular to require a Mechitza by a Bris, and rather they suffice with the men and women simply remaining on separate sides, similar to a Chuppah.[3]] Nonetheless, by the meal, a Mechitza should be present.

Pidyon Haben: Many are not accustomed to have a Mechitza by a Pidyon Haben ceremony. Nonetheless, by the meal, a Mechitza should be present, unless it is a small event for the immediate family. At the very least, there must be separate seating.

Upsherenish: The widespread custom is to not require a Mechitza by an Upshreinish event.

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[1] Yabia Omer E.H. 3:19 that although initially one should have a Mechitza by a wedding, nonetheless, Bedieved one may rely on the Heter of the Levush; Lev Avraham 1:135 regarding separate seating without Mechitza; Bnei Banim 1:35 even by mixed seating;  Sova Semachos 4:15; See Hiskashrus 588:11

Mixed seating: Some Poskim rule that even Bedieved it is forbidden to attend a wedding with both men and women sitting on the same table. [Lev Avraham 1:135; Teshuvos Vehanhagos 651] However, see Bnei Banim 1:35

[2] See Rama 264:1 regarding woman not being a Sandek; Maharil Hilchos Mila 22 in name of Maharam; Tashbeitz Katan 397; Pischeiy Teshuvah Y.D. 289:4

[3] See Or Yisrael Gilyon 23 end of page 248