Hi, Gaze!
I don't think I have a 'favourite', and I can imagine that people might pick out the most 'fun' ones, so I thought I'd mention something with a different purpose.
Remembrance Sunday, held mid-November in the UK to commemorate the sacrifices of war, demands a sombre, reflective tone, 2 minutes of silence nationwide and contemplation, and altogether just feels a bit graver and more powerful than the others. People wear red poppies in their lapels, or white if they're pacifists, and the whole thing is a reminder of the brutality that human beings are capable of... but also the heroism. It's one of the most 'mature' observance, in my opinion. The American versions are a bit more militaristic in tone.
Other than that, I always enjoy the stuff going on around the Edinburgh Festival (comedy festival) in August.