Translation:In the evening
As EllisV says, gyda'r nos is a widely used set phrase for both 'evening' and 'in the evening'.
You will find examples of its use if you look on the internet and if you look up 'evening' in a dictionary.
You will come across a lot of set expressions in Welsh that do not have literal English translations. There are also many dialect expressions from regions all over Wales, but this is not one of them.
Nor are the several Welsh dialects 'weird'!
Interesting stuff about gyda'r nos and gyda. Thank you. It looks a little - though - as if some contributors have not realised just how fluid language is and how much it evolves to meet the needs of its speakers. Also, how much it borrows from other languages to keep pace with - as a really good example - developments in science and technology. All languages are different in their written and spoken forms; Welsh no more than most. French, for example, is still pretty formal in writing (though that's changing a great deal too), whereas the French of the streets or spoken French far far less so. Surely the important thing is to be able to make yourself understood, rather than dwelling too much on detail.