I'll bet your problem is your Christmas lights, they are wet. When you reset or replaced your GFCI and you still had your Christmas lights plugged into the GFCI, it tripped the GFCI instantly. Unplug your outside Christams lights and reset the GFCI again and the test the GFCI outlet with a non- contact voltage tester or outlet tester. Problem fixed.
A set of wet Christams lights will trip a GFCI.