Is there a list of important dates anywhere? It's a little difficult hunting through all the books for them, and I think some might not even be listed in the fifth edition.
What I'm looking for specifically is...
Local Tribunals, Grand Tribunals, Tremere Decennials, The Gatherings of Twelve Years, The [Verditius] Contests, and similar things.
Not just the years - I can find those - but also the time of year. At least which season they occur in. And what happens if a few of these coincide?
Tribunals:
Rhine - "begins on the eve of the 1st of May (Walpurgis Night)" (GOTF p. 24)
Novgorod - can't find it.
Stonehenge - held one year later than everybody else's since 1201 (HtM pp. 24, 116)
Loch Leglean - doesn't say (LotN p. 34)
Rome - doesn't say (Rome p.60)
Iberian - can't find it.
Levant - doesn't say (B&S p. 100)
Provence - doesn't (Midsummer Night's Dream, p. 16), can't find anything in Mistridge, I don't have the 3rd ed. Covenants.
Grand Tribunals - I assume it supercedes the Rhine Tribunal
Tremere Decennials - can't find it.
The Gatherings of Twelve Years - "begins on the night of the full moon closest to the summer solstice" (HoH:MC p. 16)
The [Verditius] Contests - "begins on Midsummer Eve" (HoH:MC p. 116)
Whatever you want to happen in your saga. 