Why is "el" required in one sentence, but not the other?

[Liquinn3]

Hi, why is "el" required in the first sentence but not the other sentence?

"Mi día favorito es el viernes"
"Hoy es viernes, mi día favorito"

[JPablo]

The first one it refers to the "model" of day that is your preferred one. Every "Friday" is your best day. Similar to say "the average Spanish student is..." "el estudiante promedio de español es..."

The second don't use "el" (the) as you are not referring to "all the Fridays" but just to the specific situation that today it's Friday...

And talking about Friday... Have read Robinson Crusoe? (If your answer is "Yes" you will know why I jumped to this other subject... if you haven't, then, you may think I am a bit non-sequitur...)

At any rate, hope it helps...