The outdoor 9/11 Memorial — the two reflecting pools set in the footprints of the Twin Towers, ringed with bronze parapets inscribed with victims' names — is free and open to the public daily, no ticket required, with hours of roughly 8am–8pm. This is distinct from the 9/11 Memorial Museum, the underground museum built around surviving structural remnants and artifacts, which requires a paid, timed-entry ticket — the standard adult ticket runs in the mid-to-high $30s; guided-tour packages that bundle admission cost more. Museum hours are generally Wednesday–Monday, 9am–7pm, with modified hours around the September 11th anniversary.
Because the memorial plaza is free, many visitors budget a stop there even if skipping the paid museum — it's a meaningful, unhurried space to walk through regardless of ticket status. For the museum, book timed tickets online in advance, allow at least 1.5–2 hours for a full visit, and expect airport-style security screening at the entrance.