A special astronomical property allows brooms to be stood on end during particular days of the year.
In recent years the long-running myth about the equinox being a day possessed of a special property which allows eggs to be balanced on their ends has grown to encompass brooms as well. And the broom-balancing myth has likewise grown to encompass days other than the equinox on which - according to NASA, supposedly - 'gravitational pull' and/or a 'planetary alignment' creates just the right conditions for brooms to stand on their ends unsupported: Okay so NASA said today was the only day a broom can stand up on its own because of the gravitational pull...I didn't believe it at first but OMG! 😭😭😭😭😭 pic.twitter.com/M0HCeemyGt - mk (@mikaiylaaaaa) February 10, 2020 As with the egg-balancing myth, this broom-standing phenomenon has nothing to do with equinoxes or planetary alignments. With some practice and patience, anyone can make a broom (or an egg) stand on its end any day of the year. As explained in this video, a broom typically has a low center of gravity positioned just above the bristles. So if you stand a broom on end and spread the bristles just right, you can form a tripod-like structure which will hold up the broom: 
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