These celestial events conveniently come four times a year, ideal for scheduling tasks that need to happen every 90 days or thereabouts.
Change your furnace filter. Follow manufacturer's instructions of course, but a 90 day replacement cycle is pretty typical.
Shred any excess paper. Open all of the envelopes, even the ones that you know are junk mail. Separate the things you need to keep from the things that can go away.
Change your toothbrush. Do that more frequently if you have been sick, or if the bristles are frayed, but generally a three month cycle for swapping out your toothbrush looks like the received wisdom.
I am sure there are more things that can be tagged with this - "spring cleaning" comes to mind for the vernal equinox, and the winter solstice is smack in the middle of the holiday season.
