Dear Dan,
That for me is a very conditional answer. If I am here in Southwest Washington where I was born, the Pacific raincoast, then no doubt summer...all three days of it, coming usually in August; pure sunshine no rain, maybe 80*F (this is local joke, our three days of summer).
If it's Arizona, where i camped in the Sonoran Desert over Christmas/New Year once, then def winter; temps down to 32*F freezing at night, but clear every day and the solar sun quickly warms your tent up to 75*F, where the daytime temperature peaks!