The Kyoto Ancient Culture Preservation Society (homepage in Japanese here) has organized special limited time exhibitions at several temples and shrines around Kyoto for this week. Ninna-ji is included among them and for the next few days you can gain admission to Kannon-do and Kon-do! Normally these buildings cannot be entered by visitors, so it is a special opportunity to be able go inside. You can enter the buildings until May 8th with a ticket being sold near the buildings. They are open from 9am to 4pm daily.
The hours and days remain the same for the other participating locations around Kyoto, a list of which you can find here (in Japanese).