We've mentioned it before, but Mo Versi of HTC's American division is a good dude. Any given day you can find him on Twitter, answering questions from HTC customers and giving out juicy info on the next major software releases from the company. If every Android manufacturer had someone like Mo, the world would be a better place... and maybe our comment section wouldn't seem so bitter and despondent. Anyway, his latest tidbit is that the AT&T variants of the HTC One M8 and One M9 will be updated to Marshmallow starting tomorrow.
HTC One (M8 & M9) AT&T owners! Thank you for your patience. Marshmallow OS is approved, OTA to start tomorrow!!
— Mo Versi (@moversi) May 11, 2016
Unlocked and carrier variants of both phones have been getting updated to the latest public release of Android somewhat sporadically over the last few months. AT&T isn't inexcusably late to the party, but at least some versions were updated as early as January, so this isn't a sterling example of haste, either. As always, expect the updates for both the One M8 and One M9 to come out in staged waves, so it might be a week or more before the over-the-air file is available for your specific phone.