"I'm going on holiday."

Translation: Dw i'n mynd ar wyliau.

[RowenaJane]

I thought it was also correct to say 'ar y wyliau'------?

[ibisc]

...mynd ar wyliau is usual for 'going on holiday'.

You would tend to use ...mynd ar fy ngwyliau, say, for 'going on my holidays'.