Leap Year 2020
So the spring and fall equinoxes run later than the 21st. As a rule of thumb they fall on the 21st day of the month.
The year is about 365 and 1/4 days. Every fourth year is a Leap year (Feb 29 is added), this should reset the equinoxes to the 21 day of the month.
So the answear is I didn't use a calendar. But then the people printing calendars have to get this information from somewhere? Alamacs??
If this is confusing, have a glass of wine and repeat.