Resources included online minority language newsletters showing months in their mastheads, articles, localization tables (a gold mine), cold contacts with university linguistics departments, YouTube videos, minority language souvenir calendars, Wikipedia pages (especially their foreign language versions), Peace Corps language primers, tourist phrasebooks, and questions and answers posted in online forums.
Dictionaries? Yes and no. We deliberately short shrifted most of the major languages in favor of the exotic, endangered, or just plain dead and buried. Most lexicons for those, such as they exist, aren’t anywhere big enough to accommodate such minutia as Gregorian month names. Or maybe one will throw you a bone for February and April and you’ll need to use those to hopscotch to other sources. Or maybe this other dictionary is just one-way, Abkhaz to Russian, say, and while you may know Russian months, you have three hundred pages of un-selectable image scans from which to pick them out. Ready, set, go.
Certain underlying abilities do help out, like those to read and use non-Roman scripts and transcribe into or out of them. A substantial knowledge base also comes in handy, as you can think of more angles for …