They start low in January, but pick up a tad up to the end of March, drop slightly at the start of April (start of Q2), grow more until June, stay steady till the end of June, then drop by about 5-10% at the start of July (start of Q3), they then grow at a steady rate to the end of September, at the start of October they again take a little hit, but then start rapidly going up to the sales season at Christmas.
Though that all depends, some higher end MCNs will spread their ads out wisely so the CPM stays mostly the same (not too low in January but not as high in December).