In short, hot cross buns mark the end of the christian LENT period (several fasting days to be observed and giving up something for the Lent period and giving the money to the poor). LENT starts on the day after Shrove Tuesday (Pancake Day). The cross on the bun signifies the crucifixtion of Jesus on the cross. Hot Cross buns were not available until Good Friday in my day, and were freshly baked at any local bakery, even in London where I am from. They actually cost one old penny each, hence the rhyme "Hot Cross Buns, Hot Cross Buns, One a penny, two a penny, Hot Cross Buns". Good Friday at 3pm, is when Jesus was crucified on the cross. Christians will attend the solemn ceremony of the crucifixtion at churches around the world. All this came about in the 4th Century.
Unfortunately, commercialism takes over as usual and Hot Cross Buns are available just after Christmas! Whether people believe in the Christian faith or not, its always good to know why these things exist.