Hi @marjeta,

For pages, I believe this would all be a one-at-a-time thing right now :(.  For assignments, there is a bulk date editor, but no such thing exists for pages at this time.  You could certainly submit that as an idea for Instructure to consider adding to Canvas in the future.  Since pages really are just getting more date-based functionality now, it might be a good time to submit this idea so it gets some visibility.

Hope this helps!

-Chris