Because the museum gets so many visitors (it has hosted as many as 10,000 people in a single day), passes specifying a visit time (in 15-min. intervals) are required in March through August, the busiest months. You can also obtain same-day passes by arriving early and standing in line on the 14th Street side of the museum’s alley, where staff distribute passes starting at 9:45am. (If there’s no line, head inside to the information desk.) The museum also offers a limited number of same-day passes online, starting at 6am on the day you hope to visit; or you can reserve as many as 55 tickets in advance at www.etix.com (search for “Holocaust”; you can also access the online etix system via the USHMM website), for $1 per pass (you print your own tickets). Passes are valid for entry within a 1-hour time frame from the time stamped on your pass. Note: Passes are for the museum’s permanent exhibition on the first three floors. No passes are needed to see the lower-level exhibits.