Outside electrical box keeps tripping no matter how many lights I disconnect from it. Checked circuit breakers... none broken.

Obviously there is a short in the actual line to the electrical box. Anytime an electrical line is subjected to hot or cold or wet conditions expect a line failure. You may need to totally rewire this line. Or you could take a voltmeter and test the line for continuity.

Most likely it is a GFI breaker, with a test button, or a GFI receptacle, with a test and reset button, that is tripping.

These trip often when decoration lighting get wet.

What trips, and does it trip with nothing? One set? Etc?

So by your description we can assume this is an GFCI receptacle, right? If so it may be doing the job it was designed to do. Open the receptacle, check all connections, blow a hair dryer inside the box and GFCI for about 5 minutes. How is it now?

Unplug Everything on that circuit including multiple outlet plug ins.
Usually shorts occur in boxes,
Not likely you have to replace the line.

Obviously there is a short in the actual line to the electrical box. Anytime an electrical line is subjected to hot or cold or wet conditions expect a line failure. You may need to totally rewire this line. Or you could take a voltmeter and test the line for continuity.You advice this because a GFI is tripping?? No offense but this is bad advice. Hot and cold conditions do not promote "line failures" (whatever that means), nor does the circuit have to be "re-wired".

Considering the lack of real details in the OP's post it is either something that is plugged in that is tripping the GFI or a bad GFI device.

I agree with Stan about twinkies post. Jumping right to a rewire is ridiculous without additional information and he's right about the weather too.

This GFI recep could be bad, as suggested earlier, but most likely a recep down the line being protected by this GFI recep could be getting wet inside & shorting out. A good course of action is to flip the breaker off & pull the receps out of the box downline from the GFI to see if there's any signs of water.