This from the coopculture website:
"The release for sale of tickets for entrances and services to the Colosseum Archaeological Park is automatic, gradual and progressive, on an hourly basis with respect to the time of the single time slot, according to the following methods:
- the monthly release is activated 720 hours (24h x 30d) before each single time slot.
- the weekly release is activated 168 hours (24h x 7d) before each single time slot.
- the daily release is activated 12 hours before each single time slot.
For the Full Experience Underground and Arena ticket there are monthly and weekly releases."
It's my understand that the tickets release by time slot so, for example, tickets for a 10:00 AM time slot would release at 10:00 AM ROME TIME for a date 30 days from today. As stated above, there are also ticket releases for corresponding time slots 7 days before, and daily for tickets that don't include the underground and arena. In any case, it can mean being up in the middle of the night to snag them when they become available.