Banner CPM Trends - End of the Month Earnings Boost

Anyone ever notice a boost in earnings from Banner CPM programs around the end of the month? For the last year or so I've been tracking my CPM programs and found a few trends worth noting:

1. The first few days of the month are crap. I see CPM's in the 50-70 cents range.

2. By mid-month I'm seeing CPM numbers well over $1

3. By month-end I'm earning big bucks with $1.50-$2.00 CPMs.

My current CPM partner (TribalFusion) also allows advertisers to directly run ads on my site, which often results in a much higher payout ($3-$4 CPM). For some odd reason those also seem to hit my site near the end of the month.

My guess at an explanation is excess inventory. I figure most advertisers set a monthly budget, and come the end of the month you can bet the ad companies are going to make sure they hit that budget... so they start dumping all the ad inventory. Which in turn means less charity ads showing, and more paying ads... =)

So now the trick is to find which programs (affiliate, PPC, etc) pay better at the beginning of the month, and start to rotate my ad space based on optimal payout trends... Okay maybe I'm getting a bit carried away... but it might be worth experimenting with...