Tickets are released 3 separate times for the same dates. The bulk is released once per quarter for the following 3 months. The next batch is released once per month for the following month. The last batch is released once per week for the following week.
The release date for each quarter varies, but it is usually about 1-2 months before the start of the quarter. For example, for the quarter we are currently in (April, May & June), tickets were released on February 25th of this year.
The release date for each month is the first Monday of the previous month. For example, on Monday, May 6th at 9:00 am local time; they will release tickets for June.
The release date for each week is the Monday of the week prior. For example, this coming Monday, the 29th of April at 9:00 am local time; they will release tickets for the week of May 5th to May 11th.
Their reasoning behind this is to prevent hoarding in advance. There are numerous agencies out there that will have dozens of people all purchasing tickets as soon as they are released so that the agency has a stockpile from which to sell upgraded tours from a standard admission to the VIP admission. They are large companies that know that they will have no problem finding enough clients to sell out the tickets. Whereas, a small boutique tour company like mine, can't afford to randomly purchase tickets in advance; for fear of not finding enough clients for all of the tickets and then having to eat the cost of any extras. So we and others like us, only purchase these kinds of tickets when we have a confirmed client with deposit paid.
The other reason for them doing this, is to also accommodate those who do not plan in advance or are taking last-minute trips. This way there are still tickets available for them.
Please note, that you need to be online and ready to go by 9:00 am local time in Rome, on the release dates. Even then, you might not get any. It is no different than trying to get tickets for the hottest concert of the year. In fact, on numerous occasions, we have been online at 9:00 and have not been able to secure all of the tickets that we need to secure for our clients. They sell out that quickly!!!
In closing, I truly feel that the Underground is more hype than reality. That hype comes from the fact that the Underground has been open to limited VIP access for a number of years now. Because of this, it has been written up in article after article and blog after blog, as well as numerous guide books. Therefore, everyone wants to do the Underground but it is not possible as the number of people that get access to it and to the Belvedere (Panoramic) is very restricted. The Colosseum gets over 5 million paid visitors a year and only about 2% of those visitors will get access to the Hypogeum (underground) or Belvedere. So if you don't manage to get tickets, it is not the end of the world. You will be in the same boat as 98% of everyone else. You will still get to see the Colosseum and still be amazed by it!!!
While I enjoy going into the underground I actually prefer the Belvedere. Reason being is that there is not a whole lot to see in the underground. It is interesting and worth visiting but there is just not that much there. Whereas, I love the almost “aerial” aspect of the Belvedere. The views out over the city and most importantly the view over the Colosseum is just stunning. It almost, but not quite, gives you a drone-like view of the building. The reason why the hype does not exist for the Belvedere is the simple fact that the VIP access has only existed for this area for about a year and a half! It is so new that there has not been as much written up about it and therefore not as much of a demand. That is why the tickets are easier to get than those for the underground.