*Set "Minimum Stay" = 7
Go to SETTINGS->BOOKING PAGE->PAGE DESIGN and set "Default Number of Nights" = 7 and "Minimum Number of Nights" = 7
If you use weekly and nightly rates together the system will apply the weekly rate for the first seven nights and use then the nightly rate.
Go to SETTINGS->PROPERTIES->ROOMS->DAILY PRICES and set "Min Stay" = 7
Go to SETTINGS->BOOKING PAGE->PAGE DESIGN and set "Default Number of Nights" = 7 and "Minimum Number of Nights" = 7