Beside the standard schedule "Final cleaning (cleaned & ready for next guest)" , you can also add custom scheduled tasks such as change of sheets or "midweek" cleanings of rooms under "Settings > Activities and cleaning". See below.
The new scheduled task "Sheet change", created above, will display under "Units" as an empty column. For each day that a sheet change is scheduled for, an icon will appear in the "Sheet change" column, see image below. The cleaning staff can click on the icon when the sheets are changed and the icon will change status.