What is the breakdown of Κυτιακή?
κύριος = lord, master, mister
κυριακός = of the lord etc.
η κυριακή ημέρα = the lord's day
What's the difference between μέρα and ημέρα?
They both mean the same thing.
ημέρα is the form inherited from Ancient Greek, μέρα a natural development through the centuries in the colloquial language, but nowadays both words are used side-by-side.
I understand "η ΚυριΕκή". Is the voice correct?
It's not an "E", but it's not as clear an "A" as it could be, either. TTS being TTS :p
Ευχαριστώ! So my ears aren't completely off :)
It is correct to say "the Sunday after Christmas is a holiday"
That's true - when we are qualifying the day, as with "(the Sunday) after Christmas" or "the last Sunday of my holidays".
A bit like "He's not the John I used to know" takes an article when "John" is qualified with an additional phrase.
I noticed that Monday through Friday did not use the definite article in the previous lesson, but Sunday has one.
There's nothing special about Sunday as a word. Subjects (words that function as the subject of the sentence, I mean) in Greek almost always need an article.