If you are looking to enable overnight rentals for a specific price, you must first enable autosave, as conditional pricing requires autosave leads to be turned on. To do so, go to Settings -> General Preferences -> scroll down and select Autosave Leads and Admin Autosave:
Once you have turned these on, save and close.
Conditional pricing is based on Filters. Settings -> Filters.
To create a filter for an overnight rental, you'll first have to Add Filter. Enter Filter Name -> Filter Type is Pricing. Field Name is Lead Number Of Nights -> equals -> value 1 -> Add
Ex:
Save and close
To add the overnight filter, go to Settings -> Inventory, select the inventory item in which you would like to add the overnight filter to -> Pricing (If not already automatically pulled up, select the Show Conditional Pricing option). Once in conditional pricing. Behavior is Add to Base Price. Filter is Overnight. Price Name is an amount you want to charge additionally for overnight rentals.
In the example provided below, we chose $75, which is in addition to the price of the rental item and any fees associated to that item, as this is for an overnight charge.
Once this filter has been set to the inventory item, save and close.
You may have to do this to each inventory item you have.
To verify this filter is working, you can go to Leads/Events, add lead time and date (for overnight rental, price after 24 hours) and the price should auto-calculate the new price. Ex: 
If it auto-calculates the price, you may change or ignore this change. Changing this will cause the overnight filter to go into effect. If you ignore this change, you will not be charging an overnight filter to the customer.