With the NFL’s free agent negotiation period set to begin Saturday, and the signing period commencing at 4 p.m. the following Tuesday, the salary-cap strapped Washington Redskins enter a crucial week as they attempt to upgrade their roster and build on their first playoff appearance in five years. The NFL notified its 32 teams Thursday night that the salary cap for the 2013 season is set at $123 million. For the Redskins, the figure is expected to be reduced by $18 million, the amount docked by the league as the second half of a two-year, $36 million penalty for how the team structured contracts during the uncapped 2010 season. After factoring in the penalty, Washington is believed to be $3 million over the cap. Now that the team knows exactly where it stands financially, General Manager Bruce Allen and Eric Schaffer, the team’s vice president of football administration, have a fair amount of finagling to do. They have until March 12 to get below the cap for the start of free agency by either restructuring contracts or releasing players. But the Redskins need to trim more than the $3 million required to get under the cap. They must create additional spending room to re-sign a number of their 19 players with expiring contracts as well as meet other needs through free agency. For example, in 2012 Washington’s re-signings of London Fletcher, Kory Lichtensteiger and Adam Carriker, and the free agent additions of Pierre Garcon, Josh Morgan, Brandon Meriweather, Madieu Williams and Tanard Jackson cost roughly $15 million toward the cap.