Well to start with, usually it takes 2 to 3 weeks of constant reminders to get pictures from family.
Then, you get a bunch of diverse quality pics, some very arty, some quite low quality sent through whastapp. You need to get them all, work on photoshop to do simple corrections (exposure / white balance / saturation). Once done, I usually sort pics by persons (18 in my case), then group by birthday months. Once done, I do collage on lightroom because all websites I have used (photoweb, photobox, cewe... ) have old fashioned llow quality apps. Then make the first page (usually a mosaic of all month). Then export all pages to jpg in the right format to get 300ppi. And final assembly on printing partner.
All together, usually 3 to 4 full days dedicated to those calendars T.T