I wanna visit at December, is the park still open? And do I need to bring a covid test to enter?

The park is open all year round for hiking or stargazing. If you plan to go in December it's best to dress appropriately and keep socially distanced from others outside of your traveling party. The park itself does not have a test requirement but it's best to check state covid-19 guidelines before going.

Thank you for informing, but is it mandatory to provide covid test result?

The park gets very cold so dress appropriately. Arrive before dark to find a good parking spot and location to set up for the evening