My theory is Because it means day. The way this is set up is if it were saying in English Day Fri and Day Satur. I may be wrong though but that is just how I had interpreted it.
Why the first 'Dydd' is pronounced "div" and the second one "did". Also, why the first letter in every word in this sentence is in capitals ?
On an unrelated note, is there no "Idioms and Proverbs," "Flirting," and "Christmas" bonus skills in the Welsh course? I have yet to encounter them as well as the immersion tab