Except Sundays, Germany has 9 statutory holidays in all of its 16 states.¶
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In the preceding paragraph, which for some reason I can't edit, you need 'an' before 'exercise', and I think it is more customary to say 'an overview of' rather than 'about'.

If the 9 holidays are common to all 16 states, then you could make that clearer, although by saying 'in all' rather than 'in each', I think you have perhaps captured that meaning. I would still clarify with '...holidays that are common to all of its 16...

I don't understand how Sundays are an exception to the statutory holidays. I think in North America Sundays are irrelevant to statutory holidays, but you could mention them as a separate partial holiday for many people, because businesses are closed on Sunday (if that's true).

Every Sstate can add holidays, depending on the state parliament's values, which can be religious or politicial.

I think I might say 'priorities' or 'preferences' rather than 'values'. Values isn't wrong, though; it's just unusual in the context of a parliament.

UntilAs of now, Bayern has the most holidays with, 13 in total, which includeing the 9 statutory holidays.

Referring to when something happened in the past is something that both Germans and English-speakers get wrong in the other language. I haven't worked out whether there is some principle involved, that we could just learn. We don't ever say 'until now'; that much is definite.

If you mean that the number of holidays in the various states changes from time to time, then 'as of now' is correct, since it implies that Bavaria's status could change.

It's not wrong to put a common between 'total' and 'including', but in a short sentence like this, I think the meaning is clearer without it. I might say instead '...13 if you include the 9...'

I want to write a short overview about the holidays in Germany, which will be an exercise forin relative clauses and participles.

"an exercise for relative clauses" would mean that the relative clauses will be doing the exercise, not that the exercise consists of using relative clauses.

Until nowCurrently, Bayern has the most holidays with 13 in total, which includes the 9nine statutory holidays.

"includes" relates to "total", which is singular. Numbers smaller than ten are usually spelled out.

ExceptNot including Sundays, Germany has 9 statutory holidays in all of its 16 states.

"Except" sounds like Sundays are being excluded somehow, which doesn't seem right.

Every Sstate can add holidays depending on the state parliament's values, which can be religious or politicial in nature.

Or: "which can be based on religion or politics." The phrase as it was originally seems like it is missing something.

Until nowCurrently, Bayern has the most holidays with 13 in total, which include the 9 statutory holidays.

"Until now" implies that something has now changed, and we would thus also need the past tense.

Except on Sundays, Germany has 9 statutory holidays in all of its 16 states.

Every Sstate can add holidays depending on the state parliament's values, which can be religious or politicial.

UntilAs of now now, Bayern has the most holidays with 13 in total, which include the 9 statutory holidays.

Every Sstate can add holidays depending on the state parliament's values, which can be religious or politicial.

UntilAs of now, Bayern has the most holidays with 13 in total, which include the 9 statutory holidays.

"Until now" implies a change has happened and that you're expressing how something used to be. In this case, it would mean that Bayern had the most holidays, but another state now has more. As of now seems more natural, as it tells you how things stand at this moment