Is it somewhere in the official sourcebooks of 5th Edition mentioned the years of the regional Tribunals?
The Grand Tribunal is held every 33 years. Some books mention Grand Tribunals in
1228, 1261, and 1294...
Regional Tribunals are held each seven years, also there are references of tribunals helds in:
1200, 1207, 1214, 1221...
Also note that regional tribunals are added/rescheduled for the year prior to a Grand Tribunal to assemble delegations.
1227, 1260, 1293...
And of course you can build your own timeline.
Personally I place a few regional tribunals at different years and seasons from each other.
Each troupe must determine a tribunals duration and time of year. For example I suppose Autumn is a nice time to travel in medieval Europe (less rain, more food). But in southern Europe, Spring is also good.
Also note some tribunals have a particular timeline, like Loch Leglean[cite?] and you can schedule your regional tribunals as you deem better fits your saga.