At is used to indicate specific points in time such as noon, midnight, or holidays. On is used to indicate days of the week or dates. In is used to indicate months, years, seasons, centuries, periods of the day. Exceptions include using "at" for weekends and nights. Other prepositions like ago, in, after, before, till/until, from...to, since, for, during, and while are used to indicate various relationships involving time.
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