If you participate in Booking.com, you will need to contact them before the end of the year to update your account to allow or NOT allow same-day booking.
Depending on how your account was set up, you will have the same setting for either one year or two years. If you're not sure how it was set up, you must call in December to make sure it's set up the way you want it to be for 2019.
If you're not sure of who to call, or can't locate your rep's contact info, Booking.com gives this advice:
"Got an urgent question and need to call us? Then call the dedicated phone number* found under the Extranet's Inbox tab.
For questions about reservations, press "1" to reach the customer service team. For any other issues, follow the voice prompts to get to the right department.
If you can't log on to the Extranet, contact one of our teams using this directory."
Just make sure you take this step in December so you aren't unprepared in 2019.