I’m probably asking this a bit late as I’ve already sent the message, but what would be the best way to say “I hope you have a lovely day”?
I put:
“Dw i’n gobeithio bod diwrnod hyfryd iawn gyda chi” (it’s for my Nain, hence the ‘chi’ construct)
but should I have put:
“Dw i’n gobeithio bo’ chi’n cael diwrnod hyfryd iawn”??
I remember this coming up fairly recently on the old forum, and in that thread, I remember Aran saying that “cael diwrnod hyfryd” wouldn’t make sense in Welsh. He suggested something like “Mwynha dy diwrnod” instead. Not sure about your first one though.
I guess “Joio dy ddiwrnod” would’ve been simpler really. Ah well… I thought I would check anyway for if I send a similar message next time! Diolch! 
Joiwch dy ddiwrnod if you want to be formal about it.
With Jon on the cael vs gyda chi. Gyda is for somethign you possess. When you’re speaking, just go with the flow, but if you’re writing and strting to think “is this right”, change “hav” for “possess” and see if it works:
I have a cat - I possess a cat - gyda
Have a good day - Possess a good day - doesn;t work, not gyda.
I’m having a baby (I’m not by the way) - I’m possessing a baby - deosn’t work, not gyda.
Like I say, this is a thinking process, so only use iot when you have time, when writing etc, not in conversation where it will slow you right down, and people will understand if you use the “wrong” one.
Ah… Joiwch! Didn’t think of that one! Well my Nain doesn’t speak much Welsh these days, but she knows I’m learning so I’m sure I will be forgiven! Diolch yn fawr for the replies though
Just to add my bit, I would use the cael version,
“Dw i’n gobeithio bo’ chi’n cael diwrnod hyfryd iawn”
Interesting. I hadn’t noticed this exchange. I always think of gyda as ‘with’. No one owns me, but might well siarad Cymraeg gyda fi!!
Yes? No? @aran what do you think?
What Iestyn said… 
Gyda in the south is used (as one example) where we’d say ‘gen i’ in the north, so yes, possession. But as you note, it’s also used where we’d say ‘efo’ in the north…
I would have been tempted to replace the “bod” with “bydd”, in the sense “hope you will have a good day”.
Would that have been wrong?
No - in fact, it would have been a bit more natural…
To @AnthonyCusack Penblwydd Hapus i ti!! 
I wasn’t sure where to put this message, but this old thread seemed to fit!
I wish you a lovely day and successful year!
Diolch yn fawr iawn!