Technically guests have to cancel ADRs the day before to avoid paying the no show fee (e.g., so cancel on Monday for a Tuesday ADR).
In terms of making ADRs, yes, you can make same day ADRs. They just modified the system so you can make ADRs for within an hour of the current time. I did that recently at Epcot. I made a reservation for Les Chefs for dinner about an hour later (made ADR at 7:30 pm for an 8:30 pm ADR). There were times closer, but we wanted to have enough time to walk through Epcot without feeling rushed.
Whether or not an ADR for the specific restaurant you want and/or at the time you want will come available will vary by restaurant, time, time of the year, etc. If there is a specific place you want and/or a specific time, I would keep checking for ADRs from now until the day of the meal. You never know when someone will cancel an ADR.