I am not sure it will make a big difference. As these are the last three days of the markets being open, it will be busy all three days.
The 23rd has the advantage that some stalls will offer reduced prices, mainly the stalls with sweets and chocolates.
Otherwise it all depends what is easier for you to get there.
I suppose, but I´m not sure, that the 21./22. might be even more crowded because it´s a weekend.
The 23. is the last day before Christmas Eve so perhaps many people have to make preparations for the holidays.
From a houesewife´s viewpoint:-))
Both Saturday 21st and Monday 23rd will be quite a zoo in the shopping streets of the city centre. I'd expect Saturday to be even worse, though, as on Monday people will be occupied with shopping for food and such for the holidays, cleaning the house and similar *pleasant* activities. The last Saturday before Christmas is usually hell.
Shops are closed on Sunday, the markets will still be busy but you won't have the invasion of shoppers from near and far.
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