Last few years when I've bought day tickets and they usually do come out in April or so. Be warned though, last year the Saturday with Metallica sold out pretty quickly as I recall.

Also, they sell day tickets when w/e ticket sales slow up. As the camping tickets may not sell well, due to last years shitty weather, lots of w/e tickets may sell to those who were going to camp. So that may reduce the number of day tickets that are released.

I normally check the Download website twice a week starting about March, just to make sure.