Pancake Day is actually another name for Shrove Tuesday, which takes place 40 days before Easter Sunday and marks the start of Lent. In some other countries this day is called Mardi Gras, or Fat Tuesday, because it's when people ate all the good things for the last time before starting 40 days of religious fasting for Lent. Milk, eggs and oil or butter might not seem special nowadays, but hundreds of years ago they were one of the few ways of turning a basic recipe of flour and water into something richer.
As well as using all their eggs and fats before Lent, people would also go to church to confess their sins to a priest. A bell used to ring at about 11 o'clock in the morning to remind people to cook their luxury ingredients and go to confession. This bell became known as the pancake bell.