[zheshishei]

Couldn't it be solved by only taking a picture of the changed columns of the calendar instead of the entire three months?

I'm not familiar with how you're processing the picture, but it might be possible to identify the columns by utilizing the empty row at the top of each column. I count 7 positions/bits. You can use 2 bits to mark the month and the remaining 5 bits to mark the day of the month. Using this encoding, you could even add a fourth month without any problems.