Why is it just "andra veckan" and not "de andra veckan" if it is translated with "the second week"?
Andra is a somewhat special adjective that doesn't require det, den, or de before it when it is used with a definite noun.
But does that mean that you could say "de andra veckan" or does it sound weird here?
You'd use den, it's much less idiomatic but usable enough that we do accept it.
Naturligtvis! Tack så mycket!
Why isn't 'det sekunden veckan i maj' not right?
sekund means second as in 1/60th of a minute.
Varför är det inte stämmer att säga "majs andra vecka"
It's not strictly wrong, just really unidiomatic. You might encounter it rarely, but it should not be taught or accepted.
tack, du är väldigt snabb!!