i don't have any league fixtures yet but as soon as I played the 2nd leg of the cup it started showing my next fixture as a league fixture, don't know when your league opponents are decided but it's certainly at least a day before it starts
It's probably done at the same time as the season changeover - the daily update between day 1 and 2 is the same length as the normal ones at the rest of the season so it can't be done then...
Any formation or tactics advice given is based purely on experience with my teams...