New York (CNN) -- The National September 11th Memorial, set to open to the public on September 12 this year, is expecting quite a large number of visitors, if advanced ticket reservations are any indication.
Monday, reserved passes to the memorial became available on its website, and 24 hours later, 42,000 passes had been reserved, according to Joe Daniels, president and CEO of the National September 11th Memorial and Museum.
"We're super happy with the demand," Daniels said. "It's definitely still early in this, and it's pretty gratifying that everything that everybody's been working on to complete the memorial is getting to take that first step and get people to see it."
Daniels expects several million people to visit the memorial over the course of the first year, possibly as many as four million depending on how flexible people are with their schedules, as passes are reserved in advance for a specific day and time.
"I think the bottom line is, we know there is a huge demand to visit what is probably the most sacred site in the United States," Daniels said. "At the same time this memorial is going to be here forever."
Reserved passes for a specific date and time are required for entrance into the memorial for the time being due to the ongoing construction in the surrounding area, according to Michael Frazier, involved in public affairs for the 9/11 Memorial. Capacity is limited to 1,500 people at any given time and guests are encouraged to stay up to an hour in order to get as many people as possible through the site, added Frazier.
According to Daniels, entry to the memorial is -- and will remain -- free.
The museum, set to open a year after the memorial, may come with a ticket fee or suggested donation, according to Daniels. To date, the 9/11 Memorial and Museum has raised about $400 million from more than 400,000 contributors, Daniels believed. "We are going to continue to fundraise and our goal is to secure some federal funding so we can make [the museum] easily accessible for the general public to come."
According to Daniels, 176,000 people visited the memorial website in the first 24-hour period that passes were available -- more than an average two weeks worth of traffic the site usually gets.
Daniels said the site worked perfectly except for about a half an hour slow-down when the transactions initially went live and 5,000 transactions were made in that first hour.
According to its website, the memorial will be comprised of two waterfalls and reflecting pools surrounded by more than 400 trees and bronze plates inscribed with the names of the victims of the 9/11 attacks.
The memorial will be open to 9/11 victims family members on the ten year anniversary of the attacks on September 11, 2011 and officially opens to the public the next day.