Source: http://priceonomics.com/when-is-the-best...

Conventional wisdom says to buy your ticket during the two weeks before the festival-- people who no longer can go to Coachella will be desperate to resell their tickets. But the opposite could also hold-- people without tickets are at the mercy of scalpers. 

Whatever the case may be, according to SeatGeek you should buy your Coachella ticket 4-5 weeks after tickets sell out … which was about 2 weeks ago.