My crystal, your mud

The road between clarity and obscurity is paved with the refractive indices of words.

Calendar

Does anyone know how calendars are made? I don't mean the basic, workaday one where you know what day is a Saturday or a Sunday and where the important holidays are. I mean the Kalnirnays and the Panchangams and the Murugan calendar that lots of South Indian homes have, where you tear off one page a day and can't go back to last week to check something, because the paper's thin and has been crushed and discarded the moment the new day's revealed.

Today's calendar page on one of those Murugans says There Will Be Rain*.

I want to know how they knew. Presumably they made this calendar last year some time in November or thereabouts. Sure, they can figure out the status of the moon, eclipses and Rahu Kalams. But how do they know it will rain one these particular days several months in advance?

*There is.