I've been earning from Adsense for more than five years now and have some observations to share.
The best month of the year for earnings is December and the worst is August. No big surprise there, but the next best part of the year is the spring (April thru June) which is perhaps not so obvious.
The earnings per click at the beginning of each month are always higher than at the end of each month. This seems to suggest that most advertisers are on a monthly schedule starting on the 1st of each month. Towards the end of each month, budgets run out so the cost of targeting competitive keywords drops over the month.
I would therefore tentatively suggest that if you are using Adwords to promote your business and you want to keep advertizing costs down, then you make your advertizing schedule run from the middle of each month.