1st day in Porto

How do folks. With my Camino only being a couple of months away I'm thinking more and more about the logistic of my 1st day.

I already have my credential from the Confraterniry of St James in London, I'm planning on an early start on the 1st day, probably setting off around 0600/0630.

With the cathedral not being open at that time where would I find my first stamp from?