https://en.wikipedia.org/wiki/Tomb_of_the_Unknowns
Answers to your questions.
Very moving to see this in DC. Nice view of DC from there as well.
Tonight the guards stand there for two hours?!
God bless them and protect them.
"While Arlington National Cemetery is open, during the day in summer months from April 1 to September 30, the guard is changed every half hour.
During the winter months, from October 1 to March 31, the guard is changed every hour.
After the cemetery closes to the public (7 p.m. to 8 a.m. April through September, and 5 p.m. to 8 a.m. October through March), the guard is changed every 2 hours."