Fairfax House will not now open until 1 May.
The Georgian treasure on Castlegate is owned by the York Civic Trust, which said its own activities would be rescheduled until August.
The trust said today:
It is vitally important to stay connected and maintain good spirits during this testing time. Conservation work and heritage projects continue, and we thought you would be interested to see more so we will be bringing you a series of fascinating and fun videos across our digital channels.
We eagerly await the day that it is safe to welcome visitors through our doors again, however in the meantime, we hope you stay safe and send our best wishes.