Perhaps a calendar.Am I missing something ?
Also, it's been long enough, so I made this. Feel free to point people at it: https://github.com/Atmosphere-NX/Atmosphere/blob/master/docs/faq.md#what-does-june-15th-mean
(I think he has someone on the inside feeding him information about release dates and times.)
It's more of a really, really reliable gut feel for when it's the right time.