I have noticed that I make more from adsense during the last two weeks than the first two weeks of the month. Most months start out with a few poor to average days but in the last two week I make record earnings on a few days. I think this could be down to the way advertisers use adwords. It could be that they put any advertising budget they have left over into adwords. What else could explain this? BTW I have a number of sites and blogs and thousands of articles and their topics are highly varied.
You may also find the converse where the advertiser blows his stack during the first part of the month and has to idle the ads toward the end of the month. However, since there are multiple advertisers there should be an averaging so possibly they are more conservative, the surge at the beginning or end of the month has been observed by others, though.
I haven't noticed any kind of cyclical change within a month, however I have seen big swings depending on the changing of the seasons.
I've definitely noticed trends towards the first and or the last part of a month and they also seem to play out seasonally. This time of year I seem to do better in the first two weeks wheras in the summertime I do better in the later two weeks but it all tends to average out to approximately the same earnings every month.
In the second half of February I made 33% extra than the first half. In the second half of March I made 55% more than the first half... is this trend happening with you?