Before commencing the Toussaint Gwent tourny Geralt must qualify his Skellige deck with Count Monnier. If you pass qualification he’ll tell you to return in two (in-game) days for the tournament itself. If you fail qualification he’ll tell you to go out and get more Skellige deck cards by playing the locals.
If you don’t yet feel ‘battle-ready’ with the Skellige deck, go to Barnabas Basil-Foulty at Corvo Bianco and practise with him before returning. What I’d suggest is practising with Barnabas, using the Skellige deck, against all the individual factions, against his strongest deck. When you can beat those, you’re probably ready for the tournament
You can return to the tournament whenever you like; there’s no actual time limit between initiating the tourny, and actually playing it.
One last thing to note before playing the tourny: once it starts you can’t escape out of it and must play all four games, one after the other, without any losses. So be ready for that.
The Witcher 3 Guide 