Why is 0 even a number?

It is based on the properties of the even and odd numbers.

Firstly when we add an even and a odd number we always get an odd number.
Eg. 2 + 3 = 5
This property is satisfied by 0.
Even numbers when multiplied with any number produce even numbers. 0 multiplied by anything is 0.

Hence seems logical to include it in the set of even numbers.

It is required as a place holder in our place-value number system.