More news from Coopculture, in case you miss it. Free First Sundays now are a thing of past during high and shoulder season; new free days and some new free hours have been scheduled for specific months/days. From the coopculture website:
https://www.coopculture.it/en/the-colosseum.cfm
CULTURE WEEK AND FREE ENTRANCE DAYS The archeological park of the Colosseum joins the Culture Week, scheduled from March 5th to 10th 2019. During these days the access to the Colosseum and the Roman Forum/Palatine Hill will be free and without reservation, with the exception of schools, which can continue making free reservations and entering the Colosseum from the Stern Gate. The free access during the Free First Sundays of the month will be now limited to the months from January to March and from October to December.
The direction of the archeological park of the Colosseum determined the following free days without pre-sale:
9th of May
5th of June
29th of June
23rd of September
4th of October
4th of NovemberIt has moreover established the free entrance without pre-sale for the
last Thursday of the months of May – June (30th of May; 27th of June)
and September – November (26th of September; 31st of October; 28th of
November) for the last 3 hours of opening of the sites.
(Edited a typo)