Скажите, возможно ли попасть в национальный парк «Саксонская Швейцария» 21 апреля (это попадает католическая пасха)?

Судя по информации на сайте парка, сможете без проблем:

Admission to the National Park is and the famous Bastei Bridge is free. There is no official entry point, you can access the area on all paths (outside the core zone). In the core zone, you must stay on the sign-posted hiking and climbing paths as well as access paths to the rocks for climbing.