From
NHL.com - Hockey Operations Guidelines
Quote:
23-man Roster
There may be a maximum of 23 players on each Club's playing roster at any one time from the commencement of the NHL regular season through the trade deadline. Prior to the start of the season, each Club must submit to the NHL its "Opening Day Playing Roster" which shall be comprised of not more than 23 players. Each Club must have a roster of at least 20 players, composed of 18 skaters and two goaltenders. Players on Injured Reserve do not count in the 23-man limit.
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An action on the day before the regular season to submit the roster could be the event that switches from preseason to 23 man limit.
edit Actually, 12 days before regular season may be the better time to coincide with waivers...