As the previous reviews already cover the most relevant facts about visiting the Hypogeum, I would like to focus on the experience of buying the Last Minute tickets and share some tips and observations that might be beneficial to other visitors.
We were visiting Malta over New Year's Eve and as we were not sure yet on which date we could visit the Hypogeum, all of the online tickets were already gone. Worthwhile to note: In the less busy winter season the urge to book weeks or even months in advance did not seem to be that bad. Whenever I was looking up if by chance a ticket slot might open online, you could normally buy entry tickets 2-3 weeks in advance. This timeframe was unfortunately not working for us, as we were staying only slightly more than a week.
Therefore we opted to try our luck with buying a Last Minute ticket, which you can get for the noon and 4 pm tours of the following day (unless the Hypogeum would be closed on this day, e.g. because of holidays) either at Fort St Elmo in Valletta or the Gozo Museum of Archaeology. The location in Valletta seems, in general, to be more busy and therefore being on time can be important. We arrived about 45 minutes before the opening time of the ticket counter at 9 am and were lucky enough to catch the last 2 tickets for the noon tour. To be on the safe side, you should think about arriving 1 to 1,5 hours earlier in the "slower seasons" and possibly even earlier in the busy peak seasons.
Some further remarks to keep in mind
The ticket offices in both locations seem to draw from the same "pool" of open tickets and hand them out on a first-come, first-serve basis. This means that your chances of getting the ticket could be higher at the Gozo office, which in general seems to be less crowded.
Each person can buy up to 4 tickets. Considering only 20 tickets in total are available per day, this could mean in the worst case that 5 people in line before you can snatch all the remaining tickets.
Each one of the tickets costs 50 Euros and no discounts can be applied. Especially if you want to visit with kids, this is a steep price increase from the amount of 15 € (Status Quo beginning of 2024) that would normally be charged.
In the end, we were happy that we were able to secure two remaining tickets and could experience this fascinating and unique WHS. But it would have saved us for sure some stress and money if we had researched a bit better in advance and booked the tickets online.