@Joann1604 ,
as far as I know, you can't define the avalaible period. I am afraid that you have to do it the other way around and define the blocked periods, but you don#t have to do it day per day.
With your mouse you can select a bigger range of several weeks in your calendar, after this a window pops up with the selected range and you click on Blocked. Sometimes it's difficult to scroll down with the mouse to the following month without loosing the previously highlighted range, so you may need to do this in serveral steps.
If you use the multi-calander (you need to activate the professional tools to get this opition), this looks more like a very wide Excle spreadsheet and here it is easier to go the next page on the right, you can easily select multiple month and block them all with one click.