I think this is a tricky date because it’s a Wednesday, which many places are closed, but also because it is the day after Christmas! There's no reason for wednesday to be a tricky day, it's Monday the more traditional closing day. And December 26th is a regular working day in Portugal.

You'll find restaurants open in Milfontes, but don't try to do it online... go there once there. It's not a resort it's a small town that receive some tourists, not for Christmas but for new year, so you'll find a couple of restaurants open.

Of course that for some December is their holiday month, the town has "mainly" those living and working there so it's a good time to close for some rest.