Me thinks that in terms of dates
"in" refers to a measurable extent, such as an hour, a year, etc.
Example: See you for Easter in 2004!
"on" refers to the surface of a calendar.
Example: See you on Easter Monday!
"at" refers to a general point in time, such as "Christmas", "Easter", etc.
Example: See you at Easter. (general point in time)
Example: See you on Easter Day. (specific day)
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