The first practice isn't open to the public until Tuesday, but Monday is technically the start of Organized Team Activities in Green Bay.
Monday also represents the start of Phase Three in Green Bay, as recognized by the NFL's College Bargaining Agreement (CBA), which means players can practice together as a team for the first time instead of just Individual Positional Workouts (IPWs).
According to the NFL, "Teams may conduct a total of 10 days of organized team practice activity, or 'OTAs'. No live contact is permitted, but 7-on-7, 9-on-7, and 11-on-11 drills are permissible."
Even though team work is allowed for the first time, head coach Mike McCarthy alluded during the the NFL Draft that the Packers would spend more time on fundamentals and drill work during OTAs as compared to the past.
For a more detailed explanation of Phase Three and what will be taking place Monday in Green Bay, here's a passage from the NFL's CBA:
No one-on-one offense vs. defense drills are permitted (i.e., no offensive linemen vs. defensive linemen pass rush or pass protection drills, no wide receivers vs. defensive backs bump-and-run drills, and no one-on-one special teams drills involving both offense and defense are permitted). Special teams drills (e.g., kicking team vs. return team) are permitted, provided no live contact occurs. Team offense vs. team defense drills are permitted, provided no live contact occurs. Clubs may require players to wear helmets; no shells are permitted during Phase Three of the Club’s offseason workout program or any minicamp.
While the practices open to the public will be held at Clark Hinkle Field on Tuesday, Monday's session is likely to be held behind closed doors inside the Don Hutson Center.