For years, June 1 was an important date for NFL teams, because after June 1, unrestricted free agents signed by other teams didn’t count against a team’s compensatory draft-pick formula. Since 2015, that date has progressively been moved forward, and is now the Monday after the draft which this year is May 2nd.
The move was made to incentivize teams to sign veteran free agents shortly after the NFL draft instead of waiting until shortly before camp, as many teams had done in the past. The thinking was that those veterans would then be able to participate in a team’s OTAs and offseason program instead of sitting on their couch and waiting for a call in July or August.