I am trying to make sure how the Eurail Pass works these days?
Back in the late 90s I went to Europe to do some product training with IBM. I had to travel to various Cities to meet with Support reps. I found that using Eurail was more practical that flying.
I spent 400 USD to buy a month long pass which allowed me to travel around Western Europe. and meet with each region.

I am now planning a trip for my wife and myself next December/January with the focal point of the trip being the New Year concert at Vienna Philharmonic.
We do NOT fly. So we are planning to take Cunard Line, NYC to Southhampton, and then take the rail from London to Vienna, and then back to Hamburg for a trip to Ft Lauderdale, FL mid January.
With a few days to kill I'd like to make a few stops on the way from London, and on the way back to Hamburg.

Must you pick the days you plan to travel at purchase? or can you purchase the pass and then randomly select destinations as you go?