Kind of a fun way for the family to put those to use, and keep the memories.
Those calendars were notorious for not being printed fully square on the quilt Plus remember the shrink factor of the linens!
For the next ... I think the best way to do it would be to square up each calendar first. I wouldn't worry if they were all the same size. More important to get the print squared onto the piece of fabric, and thus your fabric may be off grain once the trimming is done.
Then frame each up with a good quality quilting cotton. Then join. Because of the variation from one to the other, i would do a lot of quilting to help keep each calendar stable to maintain its straight appearance as it is washed, used and loved for the memories.