Paul2000 – With something as significant as a garage closure I would suggest that too much information is much better than not enough information.
Perhaps a Notice should be placed in the Elevator, and a hand delivered Notice be issued and placed in each of the letter boxes.
As a Committee I would definitely not rely on the absentee Owners or the Real Estate Agents to pass on this information to the Tenants. That being said, in an ideal world where everybody did their job correctly, you would not need to do this.