When your customers book online they are able to add items to their basket that are across multiple dates. However, you can prevent this from being possible. If you head to Settings > Shop settings > scroll to the "Features" section > here you can see a toggle called "Restrict multi-day orders"...
If you toggle this on and "Save" to save changes, this will mean when customers book online, if they add an item to their basket for a certain date and then go to add an item to their basket that is for a different date, they will receive this error message letting them know this isn't possible; "All basket items must be on the same day. Please create a new order or clear your basket."
Please note, the "Restrict multi-day orders" feature will only apply to online orders. Not orders booked internally.