Today, the phrase "the Gods must be crazy" comes true to life. It takes a bit of craziness from That-Guy-Up-There, and all the various forms of Him Down-Here to actually make religious holidays of four different religions fall on the same day.
Today's extraordinary confluence of the sun and the moon conjure up a rare religious confluence here on Earth, namely:
How? The calendar of all these religions is as with all calendars linked with the cycle of the sun and the moon. The Islamic calendar is related entirely with the lunar cycle meaning any festival related with the moon will fall back by roughly lunar cycle of 11 months. The Hindus add an entire month {Called Adhik Mas, literally meaning "extra month"} every three years to compensate for this 11-month effect, which is why their festivals fall back and overtake the Islamic festivals. The Zoroastrian calendar is about the same. The Christian calendar (i.e. The Gregorian Calendar) stays put (except on Leap Years) and watches this musical-chairs contest, amused.
What will happen? No, nothing bad will happen. Communal riots may be expected (Especially in a country like India - Too many communities), but that is just being pessimistic. Who knows... maybe today would be the day of World Peace. Or just another festival day.
And for the atheists: Today is Friday. Rejoice! Tomorrow is Saturday, and very likely a holiday for you.