Canonically the Grand Tribunal was in 1195. HoH:TL p.13 says they're held every 33 years (with the exception of emergency and some original ones before the schedule was set). The dates count from 832.
832 The third meeting of the Grand Tribunal of the Order of Hermes. The 33 year schedule of Grand Tribunals is set. (HoH:TL p.144)
So that would be 832, 865, 898, 931, 964, 997, 1030, 1063, 1096, 1129, 1162, 1195, 2028, 1261, 1294, ...
The back of GotF explicitly lists 832 (with the same note), 865, 898, 931, 964, 997, 1030, 1063, 1096, 1129, 1162, and 1195. The back of GotF also notes the Rhine Tribunal meeting of 1194 being a year late to adjust for the Grand Tribunal (as Tribunals meet a year prior). Separately there are:
at the Grand Tribunal of 1195 (GotF p.13)
In 865 the Grand Tribunal (GotF p.16)
At the Grand Tribunal of 1030 (GotF p.18)
The Grand Tribunal of 1163 (GotF p.18)
As the Grand Tribunal of 1261 approaches (GotF p.112)
So the line looks pretty consistent. It also happens these are the same years listed in the 4th editions WGRE, which explicitly lists 1228, 1261, and 1294 as upcoming years for Grand Tribunals and also marks Grand Tribunals in 865, 898, 931, 997, 1063, 1096, and 1195.
We don't have to use those years, but it would keep things consistent with HoH:TL, GotF, and maybe some other books.