Hi Hit75 & Welcome to Lombok Lovers
December & January arent the wettest months, infact in Kuta we dont have an awful lot of rain in December. Senggigi will be wetter than the south.
Celebrating Christmas - well it is a muslim island so unless you go to the mall and see some decorations you would be hard put to know it was that time. Having said that, at Mimpi manis we do put a tree up and do a Christmas Dinner for our guests. Also Novotel in Kuta and probably the big hotels in Senggig to will have special events on Christmas Eve night and New Years Eve nights. I have been a few years but prefer somthing more low key - and cheaper these days.