- Doug, Ontario
- The start of the NHL season.
A junior-aged player who is not signed to an NHL contract by this time must return to his junior team (the official deadline is usually a day or two before the opening games). That player is essentially gone for the year. He is not eligible to return to the NHL until his junior team's season is over.
- The 10-game mark.
A junior-aged player with a contract can play up to nine NHL games as a trial period. If he is returned to junior before the tenth game, his contract is effectively put on hold: when he goes to training camp the following season, he will be in the first year of his contract.
Once the player appears in his tenth NHL game, his contract kicks in. He can still be returned to his junior club after that. But at season's end, a full contract year will expire.
- The 40-game mark.
This is when the clock starts ticking on a player's free agent status. Under the 2005 collective agreement, a player are eligible for unrestricted free agency after seven years in the NHL. But if he returns to junior hockey before playing 40 NHL games, the season does not count as an "accrued season," which means it doesn't count towards free agency eligibility.
So an 18-year-old rookie who plays at least 40 NHL games can become an unrestricted free agent at the age of 25. An 18-year-old who is returned to junior hockey before game 40 will not be eligible for free agency until he's 26. (Assuming both go on to become NHL regulars every season after that.)
Note also that a junior-aged player with an NHL contract cannot be sent to a minor pro league like the AHL unless he has already played four seasons of junior hockey. He must either stay on the NHL roster or return to junior.
There are some exceptions, particularly for players arriving from Europe. Essentially, the rules are designed to protect junior hockey players and junior teams, ensuring that NHL clubs will retain only those teenagers who are ready for the big leagues.