Depends on how many tokens you buy tbh. If you can afford to go into bidding wars, buy scouts, pay top dollar in negotiations then wait til day one of next season.
If, like me your on a budget of tokens id buy them in the last few days. Ive had success with 4/5 * players so far so ill buy high end 4* players and above these last few days, theyll either go into next season as 5* players or a few points away from 4. (level 4 is still a piece of piss btw)