The odd thing is, a quick glance at today's Radio Times S4C listing didn't mention it at all. That seems mighty odd. But on the net there's loads about it ... this link, assuming it works, shows you the way to lots of the stuff ...
cymraeg.gov.wales/DyddMiwsigCymru/?lang=en
There's all sorts of videos to watch, information about various bands that are performing ac ati. Anyhow, know you know, if you didn't already.
It seems it happens every year. But I didn't know about it. Humph.
Anyway ...
One of the songs they played a bit of was Anhrefn ... Rhedeg i Paris ... what a blast !!