Every year on the 1st of December, facebook matches millions of Dollars in donations. If you, for example, donate $20, facebook will put another $20 on top (at least for the first donations).
The event starts on Tuesday, December 1st at 8.00am EST (that is 14.00 in Berlin and 13.00 in London). The entire thing will probably only last seconds, so it is important to donate exactly at 8.00am EST.
This is an excellent opportunity to double the amount of money you donate this year (or at least make sure facebooks money goes to a top effective charity).
Read more instructions here.