Hi,
The idea is this.
Each month on the calendar is accompanied by a picture. ie 12 months, 12 pictures.
If my picture is for June, when my Mom turns the calendar to July, she will tear off the July person's picture so that she can still see my picture.
There is an unkind subtext in this paragraph.
The model who is writing speaks as if she and her Mom are proud that she is on the calendar. Yet such calendars often involve pictures of women who are naked or almost naked, which is not really something to be proud of.
Best wishes, Clive