The calendar shows a schedule of which jobs will be executed at what time and date they will be executed. This affects cron jobs which, unlike normal jobs, are not executed directly, but are scheduled and usually repeated weekly. Jobs that have already been executed are displayed in gray.
In case you want to stop jobs for a specific day because of maintenance or other things, you don't need to deactivate the jobs one by one. By holding down on a field and selecting the time in which you don't want jobs to run you can block these jobs. With this you won't have to worry about deactivating a job and forgetting to activate it again, or going through all jobs to stop them on a certain day.