How these countdowns work
Each entry above is a rule rather than a stored date. Fixed holidays such as Christmas and Halloween always land on the same calendar date, so the only question is which year. Others are positional: Thanksgiving is the fourth Thursday in November, Memorial Day is the last Monday in May, and Labor Day is the first Monday in September. Easter and the days tied to it are computed with the standard Gregorian algorithm, which means Good Friday and Mardi Gras follow it automatically.
The target for every countdown is midnight at the start of the day in your own time zone, taken from your browser. The season and equinox dates are the usual approximations rather than the exact astronomical instant, which shifts by a day between years, and the pages that use one say so. Nothing you look at here is sent anywhere.
For a date that is not on this list, such as a launch, an exam or a due date, the deadline countdown takes any target you type and keeps it in your browser. The holidays page covers the same dates in calendar form if you want to see a whole year at once.