The Days Ahead setting on the Settings > General Settings page determines how many days in advance a customer can place an individual online order. It has no impact on catering orders.
The default setting for Days Ahead is 0 days, which means that customers can only place same day orders on Brandibble.
If you would like to allow customers to order one or more days in advance (i.e. place an order on Monday for Tuesday or Wednesday) then you should set Days Ahead to something other than 0. The maximum setting is 6, which means that customers can place orders up to 6 days in advance. So if today is Monday, the latest day they can select is Sunday (that is, they cannot select the next Monday or anytime after that).