The free agency signing period begins on Wednesday, March 11 at 1 p.m. PT. This is also when trades can be made as it's the beginning of the new league year.
However, the league has a "legal tampering" period from Monday, March 9 at 9 a.m. PT until Wednesday, March 11 at 12:59 p.m. PT where teams can engage in contract negotiations with agents of players who are pending unrestricted free agents. The two-day negotiating period applies only to prospective unrestricted free agents. This is often the time that national media outlets will report on which players plan to sign with certain teams, but nothing is official yet.