Why do Christians celebrate pancake day?

Because they're delicious.

Also, because of Shrove Tuesday (AKA Pancake Tuesday, Fat Tuesday, Mardi Gras, etc.) which is the day that goes before Ash Wednesday, better known as the start of Lent. In English traditions, people ate a lot of pancakes before Lent because of the foods they're made out of (sugar, flour, fat, eggs, etc.), which people weren't allowed to eat during the Lent-fasting.