The season starts again the first week of September.
The next events on the calendar are the All-Star game, known as the Pro Bowl, on Sunday.
Then comes the fun stuff.
The free agency period, or as footie fans call it, the "Transfer Window".
This can go on from March 1st up until September, depending on who is availabe to sign. There is also the NFL Draft, whereby teams select players who have graduated from college or declared themselves eligible to be selected. This is a BIG event in the US, mainly because a great number of NFL fans also follow college football, and this is where the pipeline of young players come from.
You see quite a few trades of players and/or draft picks at this 2-day event. This happens during the 3rd or 4th weekend in April.