hanamaru
Name the days of the week
Each weekday name ends in ようび, and なんようび asks "what day?".
JLPTN5
[day]ようび + です
The seven days
The seven days all follow one pattern — げつ・か・すい・もく・きん・ど・にち + ようび (Monday to Sunday) — and なんようび asks which day it is. Answers use the same X は Y です frame you already know.
Tips and Common Traps
Maru· Common trap
なんようび asks which weekday, なんにち asks which date. They sound alike and answer different questions.