Thought I was knowledgeable about this stuff so tried to assist a friend going to Rome in late June.
He had his heart set on an Underground Tour and saw a few weeks ago no availability for his dates (or I should say only 1 ticket and they are a party of 2)
he should have purchased them months ago but is for their Honeymoon and had other things on his mind. I offered to help plan their trip itinerary as a friend since I knew they were stressed about it and I have been a few times to Italy.
Reading the website linked below a couple of weeks back I told him not to worry as it appears only a small number go available for quarterly tickets and the rest open up monthly which for June would be Monday the 7th of May. Told him to set his alarm so he could login and buy tickets at 3 AM his local time when they come available.
https://www.coopculture.it/en/ticket.cfm?office=Colosseo%2C%20Sotterranei%20%2C%20Terzo%20Ordine%20e%
20Belvedere&id=0&evento=6
Well 3 AM came and went and no tickets were added for the English tours for underground or for combo of belvedere and underground. Just Belvedere were added in English.
They did add the opportunity to buy tickets for Underground in either Italian, Spanish or French though.
I advised him to just purchase Italian or Spanish ones since the main arena you can use the audio guides they sell or Rick Steves free audio tour and the underground tour portion is not in English.
Think he is still debating that or to buy tickets from a tour company like Walks in Italy for more money and take the tour of the underground in English with them.
Did I give any bad advise? and I am still a little confused ; have to guess if it says quarterly they open them all quarterly and leave none for monthly. if it only states monthly availability than you can go buy the thought process I had.
So for other travelers ; if you want to get the Underground Tour in English be ready to purchase 3 months or more in advance right when they go on sale and don't wait.