My adsence CPC increases on 20th of every month decreases on 1st

I started my gardening website 6-7 months ago. Since last 4 month, my CPC increases up on the 20th day of every month and decreases again on the 1st day of next month.

For example, my CPC from 1st to 19 use to be around 0.11 to 0.12 $, and on the 20th of month it suddenly increases up to around 0.20 to 0.25 $. It happens every month.

There are no change in settings, in traffic, in ad unit, in CTR, there is also no increase or decrease in traffic from any country. In the essence, CPC increases on 20th day and decreases again on 1st of every month.

Why this fluctuation in CPC happens every month and how can I stable it on higher CPC.

 

It all depends on advertisers, may there is high bidding on ads during those periods.

 

When do you usually post your articles(approximate dates). Is there any link to this. Can you share your url please.