Intializing Serial crashes UNO
Since no other code is posted, I am going to guess that you're performing a loop on months[], which has 13 elements (last one is NULL). 

You are likely also trying to walk the days[] array (or some other array), which only has 12 elements.

Since days[] or the other (not pasted) array doesn't have 13 elements, you're probably overriding a critical bootloader memory which causes a crash.