Here's the DEP schedule from the last several years (except for early 2008 when it replaced the BPB in DCA from Jan-Mar):
The DEP traditionally goes dark the day after Labor Day. It then returns for a short run during the peak Christmas season (usually from the Friday prior to Christmas through the last day of Christmas peak in January -- where it runs nightly). It then goes dark again until Spring Break peak, when it returns and runs nightly the week prior to Easter. It continues running nightly through Spring Break peak season, then it runs only on weekends the remainder of April and May and early June. When peak Summer season begins in mid-June, the DEP will begin running nightly again.
Hope that clears up any confusion.