How to make entire SubSite in SharePoint Read-only

Question

Hi

I have a Site named Test
It has 3 Subsites Test1,Test2,Test3
All Subsites have 10 libraries with many folders and many content types
There are around 10 user groups on this site
I have to make the entire site read-only for all users on a particular event.

My current approch is to break inheritance of all Libraries and I am trying to assign Read Only permission

Please help

Thanks
Jiniv Thakkar