Formal season usually happens around mid-January to mid-February. There are always faculty-specific ones (Formaldehyde, Mac Eng Fireball, etc), but there's also a Res-wide formal, SOCS formal, first-year formal, and the Charity Ball, which is as close to "school-wide formal" as you can get, I think. There's no way you'll miss any of these since they're advertised like hell.
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