Why do days mutate in Welsh when standing on their own, and why does the object of a past-tense sentence mutate when following 'chi'?

Question

Background:
The Welsh Duo Lingo course was designed in two parts: the "flash cards" (the things you do in the app) and the accompanying course notes.

Unfortunately the notes were never available inside the app, so many discussions on the flash cards point to an address where a more thorough explanation of the grammar can be found.
Then, to make things worse, that link rotted, so anyone taking the Welsh Duo course is effectively learning blind.

How to restore that guidance is a separate issue, but I'm trying to persevere with what I can piece together from elsewhere (the content for about 3 of the 80+ units has been preserved here). I understand a few of the contexts where a mutation is 'triggered' in Welsh, but then a flash card came up with two mutations, neither of which I understand, which leads to my question:

Wnaethoch chi bryd o fwyd ddydd Sadwrn?

Why does 'pryd' mutate to 'bryd' and 'dydd' mutate to 'ddydd' in this sentence?

For ddydd, my best guess is that the mutation marks that it is showing when something happened (like using the proposition 'on' in English), but for bryd I'm really not sure. Is it because it's the object of the past-tense sentence? Is it simply because it follows the word 'chi'? I cannot easily apply any of the rules found here.

Perhaps the course will one day be 'fixed' and access to the notes will be restored, but hopefully this question remains self-contained and relevant enough to stand on its own.

Additional: This question is about learning the Welsh language rather than language learning in general. I couldn't find a Welsh-specific stack exchange, but if there is one, and this question would fit better there, please feel free to move it! I don't have enough reputation to add/create a welsh or welsh-language tag.

Answer 1

There are actually several mutations here in this question:

Did you make a meal on Saturday?

The first word:

gwnaethoch

This mutates in the 'soft' form (by losing the 'g') to wnaethoch, as it is at the start of a question. Without the mutation, this would be a statement:
You made a meal on Saturday.

In my continued learning, I have since found out why the next mutation happens:

bryd o fwyd

This is explained in this YouTube video:

There's always a soft mutation after the initial part of a past tense [clause]

I interpret this to mean this will happen after the subject.
In the example given, wnaethoch is the verb and chi is the subject, so whatever comes after chi takes its soft mutation. See the full linked video for more examples with subjects other than chi.

I've still not worked out why we have a mutation for Saturday:

ddydd Sadwrn

I will return and update this answer if I find out.