个 ge usage plus 每 mei
The reason is that 天, 日, 周, 年 are themselves counter words / classifiers (so they don't need an extra one), while 星期, 礼拜, 月 are not classifiers and therefore require a classifier, which happen to be 个 for the all of them.

Note that although 月 is listed with the ones that require a classifier, it's quite frequent to hear 每月 as well (which I believe makes sense since we already have a classifier for day, week and year).