I found this on the NASM website (http://airandspace.si.edu/visit/hours_extended.cfm). It is not posted on the www.si.edu Extended Calendar, yet.

The National Mall Building
in Washington, DC will be open extended hours during the Spring/Summer of 2013:

10:00 am to 7:30 pm
Every day, March 20 - April 14.
Fridays and Saturdays only, April 19 - May 18.
Every day, May 24 - September 2.

The Museum will close at 5:30 pm all other days. In addition, the Museum will close early on selected dates during the extended hours period, due to scheduled evening events. The following list of early closings will be updated as information becomes available.

Not dates for early closings were listed.