The tentative dates for the first days of offseason workouts, OTAs, and minicamp have been announced by the NFL; you can find every team's information here.
If you want the official rules, here they are directly from the NFL's mouth:
As per Article 21 of the Collective Bargaining Agreement, each club’s official, voluntary nine week offseason program is conducted in three phases
Phase One consists of the first two weeks of the program with activities limited to strength and conditioning and physical rehabilitation only.
Phase Two consists of the next three weeks of the program. On field workouts may include individual player instruction and drills as well as team practice conducted on a "separates" basis. No live contact or team offense vs. team defense drills are permitted.
Phase Three consists of the next four weeks of the program. 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 permitted.
Article 22 of the Collective Bargaining Agreement stipulates that clubs may hold one mandatory minicamp for veteran players. This minicamp, noted below, must occur during Phase Three of the offseason program. Head coaches hired after the end of the 2013 season are also entitled to conduct an additional voluntary veteran minicamp.
Each club may hold a rookie football development program for a period of seven weeks, which in 2014 may begin on May 12. During this period, no activities may be held on weekends, with the exception of one post NFL Draft rookie minicamp, which may be conducted on either the first or second weekend following the draft. The dates of the post draft rookie minicamps will be circulated at a later date.
For specific information and detailed offseason program rules, please see Articles 21 and 22 of the Collective Bargaining Agreement, available on nflmedia.com and nflcommunications.com.
Enough mumbo-jumbo. Open up your calendars... I'll hold. Here are the tentative dates for the Texans' offseason workouts:
First Day: April 7th
OTA Offseason Workouts: May 27-29, June 2-3, June 5, June 9-10, June 12-13
Voluntary Minicamp: May 6-8
Mandatory Minicamp: June 17-19