I have recently started setting up Room Resources in our Office 365 tenant and quickly discovered that that rooms are only available Monday to Friday 08:00 to 17:00 by default.
Fortunately there is a Powershell command to modify this to suit your room requirements.
First connect to Exchange PowershellSet-MailboxCalendarConfiguration -Identity -WorkingHoursStartTime 00:00:00 -WorkingHoursEndTime 23:59:59
This is for an individual room resource. Alternatively if all your rooms have the same availability schedule you can change all at once with the command below