Our Würzburg day trip overlaps with the German Catholic Convention. Bad idea?

A co-worker of mine, who has family in Wurzburg and visits at least once a month, says there's no way he'd ever go on that weekend. Best I can do. I usually look at hotel bookings; if they're sold out, or the prices are crazy high, I avoid the city. The nice thing about Wurzburg is it's an easy walk into the old town from the Bahnhof. Or just a couple stops on the tram. And it's a good bahnhof too; lot's of services.

As you exit you'll see what looks like a pile of luggage to the left; it's the memorial to those who were taken away. There's a nice park in that direction also and a Lidl market just past that. It's a good place to get food, snacks, etc without paying too much.

But you will rarely find more helpful scouts than on this event. Gut Pfad ;-)

Has anyone been in Würzburg during this time and can offer some advice?

Deutscher Katholikentag is changing place everytime.

The Katholikentag attracts around 20,000 participants, spread over four days. There will certainly be more life in the city, but there won't be unbearable crowds at the sights, because the participants are coming for the program and not for tourist reasons. One advantage of the Katholikentag is that you don't have to rely on the usual two English tours per day for the residence, but can walk around freely and ask questions to the “Ciceroni” distributed throughout the building. What I could imagine is that the cathedral will be closed at times, but the most important thing about it is the building, not the interior (lost during the war). Incidentally, the main venue for the meeting, the Congress Center, is located far outside the city; this will draw some of the visitors away from the Altstadt. The restaurants of the large wineries will certainly be fully booked, but at least in the Bürgerspital there is a wine bar where you can taste and also buy wine (the entrance is on the corner of Theaterstraße and Semmelstraße). Add some snacks from the traditional Hanselmann bakery (Hofstrasse, directly opposite the Residenz) and enjoy them in the Hofgarten behind the Residenz, and your restaurant problem is solved.

The Katholikentag attracts around 20,000 participants, spread over four days

I would expect more this year because of relevance and many things to discuss, e. g. new Pope.

there won't be unbearable crowds at the sights, because the participants are coming for the program and not for tourist reasons.

I am not sure if some sights are interesting and Catholic relevant enough to attract a few hundred or thousand visitors.

Background: Würzburg has been a bishopric since 741/42 and was granted coinage, customs, and market rights in 1030. Under Bishop Konrad I of Querfurt (1198 to 1202), the Bishopric of Würzburg began to develop into a sovereign state with Würzburg as the prince-bishop's residence. More details by diocese Würzburg in German language.

I would assume that crowds at sights are not going to be a huge issue, as the visitors will mostly stick to the program (which should not include sights, I assume) - as others have noted before.
Lockers at the train station might be a significant issue though, as a lot of visitors will come by train and could lock their stuff for the day until they can go to their accommodation in the late afternoon.
I have a hard time figuring out if it is definitely going to be bad, but it could be.
Now it is possible that you can lock it or give it to the custody of a clerk at a museum, which should be easier, but it could still be a hassle to drag it around.

Lockers at the train station might be a significant issue though, as a lot of visitors will come by train and could lock their stuff for the day until they can go to their accommodation in the late afternoon.

The Katholikentag will be setting up its own luggage storage system for day visitors at the main conference venues, and, as far as I understand, free of charge. This is unlikely to motivate many visitors to pay the high locker prices out of Christian compassion for the struggling DB.

Many older visitors also stay in (free!) private accommodation, and many younger visitors stay in shared accommodation in schools. So none of them pose a threat to the needs of normal tourists.

In principle: if you really can't find a locker at the train station, go to the tourist information office in the Falkenhaus on the market square—they won't turn anyone away.

Thank you all for this great advice! This does make me feel better. I've also reached out to a few local hotels to ask if they will store my bags for a fee (assuming the DB lockers are full).