We will be on The Best of Spain tour October 23-November 5. I keep reading how important it is to buy tickets in advance for many of the sights, especially those that require timed tickets. I have tried to purchase tickets for the Picasso Museum and La Pedrera/Sant Pau but it appears that tickets are only available through August. All of September seems blocked out as CLOSED for Picasso. Can anyone shed any light on navigating the websites or the typical open window for purchasing tickets?
Probably too early. Many sites only book a limited time in advance.
The ticket lines at some of the top sights in Barcelona are definitely to be avoided, but I've never heard that those places sell out very early. The first time slot of the day is especially popular, so it's good to grab those tickets well ahead of time if they are critical to you, but there's no need to worry at this point about tickets for October or November.
I have no special knowledge about the Picasso Museum, but I imagine it isn't going to be closed this fall. It packs them in every day; a lot of money would be lost.
The Sant Pau modernista site has twice been a simple walk-up-and-buy-ticket stop for me, as recently as May 2019. It's highly unlikely there'd be any need to buy that ticket in advance, and it's really nice to have some flexibility in your schedule. It's not easy in Barcelona.
I can confirm that the Hospital Sant Pau is really uncrowded and you can just walk up and buy your tickets at a time that works for you. We were there in the last 12 months.