HTC One (M7) on AT&T to begin receiving Android Lollipop update Tuesday, HTC One (M8) on April 6

As we have outlined already about what this update brings to the table, the HTC One flagships on AT&T will be upgraded with Android’s Material Design motif, the 64-bit ART compiler, and Project Volta to get better battery performance.
HTC has made a pretty good effort to bring update to its devices within 90-days of the major release from Google. However, Android 5.0 was not bug-free, and given that along with carrier considerations, HTC had to relent from that goal a bit. Still, HTC One customers on AT&T are a little behind the curve this time around. Better to have it done correctly rather than fast, right?
source: Mo Versi (Twitter 1, 2)