it had to have an exact date and commemorate an exact time.

On purely practical terms it is convenient to have predictable holy days. In the UK, Easter is a movable feast and few people can predict (or know what is) the vernal equinox, let alone figure the rest of the calculation. And the Gov delight in making a very predictable end of May holiday into a queenie June celebration.

Stick with a date and remember why it exists.