According to the official site of the Superintendent of Excavations, the site will be closed on May 1.
Here are the hours.
Opening times November - March, every day from 8.30 a.m. to 5 p.m. (last admission 3.30 p.m.) April - October, every day from 8.30 a.m. to 7.30 p.m. (last admission 6 p.m.)
Closed: 1st January, 1st May, 25th December
Bill Thayer probably has this bookmarked but the official site is quite comprehensive. I liked the entrance page...check it out.