Month names collection / metastudy

[Koen G]

This subject confused me the last time as well. I thought the "re" ending was supposed to be the correct form for "in", but I suppose the names are just latin. 
I still have no idea about the "magio" though.

Correct, in some cases when you see a phrase or paragraph in the vernacular, the appropriate Latin inflection may be used. It would be a bit like if we would say in modern English:

"Christmas is on the 25th Decembri." (Genitive)
"My birthday is in Iunio." (From Lat. Iunius).

English actually still does this in certain contexts, though not with the months. For example, a doctor may look at a patient's uterus (nominative) to see what's happening in utero (ablative).