Which is the best day to visit Leipzig?

I can't answer about Leipzig, having never been there myself, but it is usually easy to check the hours of large museums on their websites, which typically are quite comprehensive.

I'm not sure what museums in Berlin most interest you, but I just checked two of the museums on Museumsinseln (Pergamon and Neues) as well as the Deutsches Historisches Museum, and all are open on Mondays.

The Visitor Centers at the Berlin Wall Memorial and at the Memorial to the Murdered Jews of Europe are closed on Mondays, but the outdoor areas are open.

I hope you are not planning to make this trip without a good guide book. That will make your life much easier and will enrich your visit to Berlin, where there are so many fascinating things to see that you'll probably be kept quite busy for the full three weeks.

Hi,

Expect museums to be closed on Mondays in Germany, the same applies to Leipzig. There may be exceptions.

A few years ago I went to Leipzig as a day trip to see one museum in particular, the Völkerschlacht Denkmal and Museum, This is the huge Prussian monument completed in 1913 to commemorate the Allied victory 100 years prior over Napoleon at the the Battle of Leipzig, what the Germans call the "Battle of the Nations."

In 1945 it was US troops that got to Leipzig first, ahead of the Soviets. When the Americans reached the huge monument area, they got sniper fire. To counter that the Americans turned artillery on the monument. The S-Bahn can be taken from across the main train station, Leipzig Hbf, to reach the monument.

I've been reading online about how several of the museums lack English
explanations so I wonder if previous Leipzig visitors can tell me (a)
if the city is worth a visit and (b) are museums also closed on
Mondays in Leipzig?

Yes, yes & yes.

The Stasi Museum (former Stasi headquarters) is an example of a place to visit in which there is no English translation as you walk from room to room. That is PERFECTLY ok. On the day I went, there was no English audioguide available but typically there is one of you would like to have use of it. You will see the Stasi HQ as it was left when it stopped being operational. You see the propaganda, you see how they targeted children, it's chilling.

Here is how Lonely Planet describes it:

In the GDR the walls had ears, as is chillingly documented in this
exhibit in the former Leipzig headquarters of the East German secret
police (the Stasi), a building known as the Runde Ecke (Round Corner).
English-language audioguides (€4) aid in understanding the all-German
displays on propaganda, preposterous disguises, cunning surveillance
devices, recruitment (even among children), scent storage and other
chilling machinations that reveal the GDR’s all-out zeal when it came
to controlling, manipulating and repressing its own people.

I just googled them and they are open daily INCLUDING Mondays except around Christmas and New Years. Then there is St Thomas Church. Then the restaurants. I loved my day trip from Berlin to Leipzig.

I would check the schedule for concerts at the Gewandhaus and the Thomas Church and try to pick a day when an intersting concert is on.

The Museum Runde Ecke (Stasi HQ - English audio guides are usually available) is open daily as is the Völkerschlacht monument. The Bach museum is indeed closed on Mondays.

Leipzig, however, has a beautiful historic core that is very compact to wander around (contrary to Berlin!). There are even daily guided walking tours you could join. No need to enter a museum here. And if it's raining, head inside one of the famous passage ways such as the Maedler Passage. And don't miss the Nikolai Church where the Monday demonstrations started which eventually brought down the Wall in 1989.

Thank you for your replies everyone! We were actually in Berlin for three days last summer so am excited to plan an extended visit this time. As for Leipzig, this will be our first time there. Anyway, after researching the places we'd be visiting, I've narrowed it down to either a Tuesday or Saturday because those are the days when the Zeitgeschichtliches Forum will be conducting guided tours for the public at 2pm. I know I could go explore the place by myself but I love hearing real stories and explanations from a museum guide. I don't know if Saturday means there are more public markets open, more crowds versus Tuesday with no markets and less crowds? Someone private messaged me that Leipzig is really busy from Tuesday to Saturday but left it at that so I'm still wondering.

IMHO it doesn't really matter. Leipzig doesn't see truckloads of tourists like Prague or Florence, so both days will be fine. Tuesday means people work or shop in the city centre, Saturday means less working and more shopping. It doesn't make a huge difference in terms of crowds. It will be lively, not packed.
The farmers market in the old town is on Tuesday and Friday.

Deutsche Welle has some Leipzig suggestions, such as:
Leipzig - the better Berlin?
10 reasons for Leipzig