Thats because your earnings depend on the investments done by Marketing agencies for advertisements.
As month ends come closer, the marketing agencies have to complete a target. They are completely ready with their marketing plan. So you will see, that month end earnings are much higher then month start earnings.
As the month progresses, more and more digital agencies advertise and the maximum ROI for us bloggers happens on month end. The last 7 days are always awesome for me at least.