I have an exterior Water Resistant GFI which I have my Christmas lights plugged into. It worked all day and when it started raining the GFI tripped, and I couldn't get it to reset until I unplugged the extension cord. I have used electrical tape around all my extension cords and lights to ensure this would not happen, but it did. What do I need to do?
Your extension cord and/or holiday lighting has at least one ground fault, possibly more, based on your symptoms.
Whatever you did to "prevent" that was either not enough, or not done to the location(s) with ground fault(s)
Simple as that. You either need to swap out the part(s) with ground faults, such as damaged insulation on a light string, or prevent ground faults at cord junctions that get wet more effectively. I find that raising the point where one cord plugs into another and placing a bucket or similar waterproof item over the joined cords is far more effective than merely taping the plug to the socket, for instance. Doing that, they don't get wet there.