The National Museum of African American History and Culture remains so popular that the Smithsonian will continue requiring free advance passes more than two years after it opened.

The newly-announced policy for 2019 goes into effect in January, and it isn’t quite straightforward.

Weekend visitors will always need a timed pass to get in. On weekdays, it will depend on the month and time of day. In the “off-peak” months from September to February, open up entry will available all day. From March through August, visitors will need passes before 1 p.m. After that, the doors will be open to everyone.

The museum has tested out a couple of different options for timed passes in recent months—leaving some visitors confused. While the new rules are a little complicated, they will stay the same for the entire year.

“Communication with visitors is a top priority and something that we endeavor to do as effectively and quickly as possible,” said Shrita Hernandez, spokesperson for the National Museum of African American History and Culture, in an emailed statement.

Museum officials will re-evaluate the policy for 2020.

There’s a couple of different reasons for continuing to require passes, as Mikaela Lefrak explained in October:

The museum welcomed nearly 2.4 million visitors in 2017, its first full year of operation. It was the fourth-most visited Smithsonian institution of the year after the Air and Space Museum (7 million), the Natural History Museum (6 million) and the American History Museum (3.8 million). None of those others limit visitor numbers with a timed entry system.

Part of the reason is sheer size. While the African American museum is large—nearly 400,000 square feet spread over seven stories—it’s tiny in comparison to some of its Smithsonian brethren on the National Mall. Two-and-a-half African American museums could fit inside the Air and Space Museum.

“These legacy museums have incredible amounts of square footage,” says Linda St. Thomas, the Smithsonian’s chief spokesperson. “In order to have a good experience, [the African American museum] can’t let in quite as many people.”

The structure and content of the museum’s exhibitions have also compelled its leadership to continue limiting crowd size, St. Thomas says. Visitors often get bottlenecked in the three underground floors that chronicle the transatlantic slave trade, American slavery, and the country’s racist past. There’s a lot to read, for one, and the heaviness of the subject matter can lead visitors to linger.

TLDR? Here are the basic details of the 2019 policy:

• Weekends: No matter the month, you’ll always need a timed-entry pass to enter. They are made available in advance (released on the first Wednesday of the month for three months in advance) as well as the same-day (released at 6:30 a.m.), and both run out quickly.
• Weekdays (March-August): Visitors need a timed-entry pass to enter before 1 p.m. (see above for details). After that you can walk in without a ticket.
• Weekdays (September–February): Walk-up entry is available all day.

Advance passes are available here.