The offseason is ready to kick off. "Legal tampering" begins at midnight.
What is "legal tampering" you ask.
During that time, no actual contracts can be signed. Instead, teams can work with agents on specifics that can be made official at 4 p.m. on Tuesday. Agents are important in this time period, as contact with players on other teams is still disallowed. Any negotiation with representatives of restricted or franchised players is also forbidden.
This essentially means free agency begins at midnight. Even though no contracts can officially be signed until Tuesday, there is nothing to stop teams from agreeing to contracts that can physically be signed in a few days.
Why does the NFL allow this? Teams used to do this even when it was against the rules in the days leading up to free agency, and there was no way to police it. The path of least resistance was to allow it.