Reiho-kan, the building that houses many of the temple’s treasures including statues, scrolls, and scriptures is open to the public twice a year. The spring session lasts from April 1st to the fourth Sunday of May, this year closing on May 22nd. Many items are on display every time (such as the Heian Period Amida Triad, a National Treasure) but other items are rotated every session, so there is always something new to see!
If you cannot make it this month, the next opportunity will be the fall session, from October 1st to November 23rd. I hope that you can make it!