(CNN) — Yellowstone? Free. Yosemite? Free. Everglades? Free!Saturday is the start of National Park Week 2023. And in celebration, the National Park Service is offering free entry to all 423 of its sites on April 18. That includes not only those marquee national parks, but all other types of sites the NPS manages: National battlefields such as Antietam in Maryland; national historic sites such as President Theodore Roosevelt’s birthplace in New York; national monuments such as the Native American Effigy Mounds in Iowa; and national seashores such as Cumberland Island in Georgia.