The month of May is the only full month in the hockey calendar year where there is a decisive split between the 31 teams in the league. From October to mid-April, all 31 teams are competing. From mid-June to September, all 31 teams are making changes and preparing for next year.
But in May, there is an obvious split. While eight teams (that eventually become four and then two) continue to battle it out for the Stanley Cup, the other 23 or so are taking care of various in-house tasks. That could include hiring new general managers, presidents, or head coaches.