In the Philippines, there is a law that says Filipino employers must pay their employees a bonus that is equal to one month's salary. This law is called the 13th Month Pay Law.
This law only pertains to businesses that operate in the Philippines, so technically, you don't have to worry about it if you're just hiring a virtual assistant (and you live outside the Philippines).
However, I highly highly recommend that you pay your virtual assistant this yearly bonus. Here's why: