Ananta Samakhom Throne Hall is open every day except Monday from 10 am until 6 pm. It is closed on a number of Thai public holidays like New Years day, Songkran (April 13-15), the King’s birthday (December 5) and the Queen’s birthday (August 12).
Admission fee is 150 Thai Baht per person. If you have a ticket for the Grand Palace that is not older than 7 days, you can use that get in free. It is also valid for the Vimanmek Mansion, next to the Throne Hall.
Taking photographs inside the Throne Hall is not allowed. Appropriate clothing is required, which means long pants and long sleeved shirts for men and long dress and long sleeved shirt for women. Long dresses are available for use at the entrance that can be returned when leaving.