It’s never celebrated/observed on the same day unlike your birthday or Jesus’ birthday (I know it’s not really the 25th of December, but it’s always celebrated in the 25th).
People are just mourning Jesus’ death on a random day just because it’s Friday lol
According to the New Testament, Jesus was crucified the week of the Passover on the day before the sabbath. Passover is determined by the lunar calendar, so it moves because the lunar calendar doesn't sync up perfectly with the calendar we use now.