In any event, have some Welsh cakes!
Wait, I’ll just be putting on my raincoat and unfurling my umbrella.
OK, go ahead and say it.
Dydd Hapus Gŵyl Dewi
That’s from translate.google.com, and I cannot vouch for its accuracy.
Dydd Gŵyl Dewi Hapus!
Straight from WalesOnline.co.uk
Dang. Now I’m hankering for rarebit. 
As to how you’d SAY it: (Learning Welsh) Basic - How to say (happy) Saint David's Day - YouTube
My family is massively Welsh and I’m making rabbit (among other things) for dinner tonight. It’s my first time using sodium citrate to emulsify the cheese and I’m unreasonably excited about it.
Oh wow… so I just the other day was looking for the answer to a coding problem that has been nagging me at work. I finally found a blog post by a guy (named David) that I decided could help me with my problem and I wrote to him. He wrote me back and helped me with my problem and I was so overjoyed that I said “It’s Ash Wednesday and here is St. David to save the day!”
Never heard of the actual St. David or his day, but now I think I am going to convert to Catholicism and become Welsh.
August West’s version is correct. What you have said is: “Happy day. Saint David’s Day.”
Fun fact: gŵyl “holiday” comes from the same Latin word that gives us “vigil.”
A little late. and Welsh is not my mother tongue, but I’m pretty sure it’s pronounced “Here, have a beer!”