Purely tradition. The story goes back over 4000 years to the Egyptian times.
The god Osiris was killed by his bother, Seth, on a Friday and was brought back to life by his sister, also his wife (tut-tut), Isis, on the following Monday. This story seems to have then been hi-jacked by another well known religion and has persevered to this day.
The strange thing is the way that the date of Easter is calculated. It relates to the phases of the Moon and is extremely complicated. The general rule is that Easter Sunday is the first Sunday after the first Full Moon on or after the Spring Equinox but this is just a simple shortened version of things. In reality it is much more complex.
This means that Easter can be as early as 22 March or as late as 25 April.