Milo - depends what part of the country you will be, however, in general it is cold can be wet and windy in the north. It's winter. The beach tourist resorts are milder but the nights are cold and of course the south being dessert is freezing at night but can be warm during the day.
Lesley is right. I wouldn't go in December. Better weather over in Egypt (though still not ideal). Head a couple more hours south to The Gambia and you've hit the sunshine. Try this page.