平日 and 祝日: dates in opening-hours notices
Distinguish weekdays and public holidays from an individual's days off.
Weekdays as a category
平日 (へいじつ) usually means weekdays as distinguished from weekends and public holidays. 「平日は九時に開きます」 says opening time is nine on weekdays.
When the exact scope matters, check the days and exceptions stated in the notice. The word alone does not reveal an individual's work schedule.
Public holidays
祝日 (しゅくじつ) means a public holiday. 「祝日は休みです」 can announce that a shop or facility is closed on public holidays.
「祝日でも営業しています」 says it is open even on public holidays. でも does not exclude those days from opening.
Reading the conditions
「平日のみ利用できます」 restricts use to weekdays. のみ narrows the permitted range.
「月曜日が祝日の場合は休みます」 says it closes if Monday is a public holiday. Do not expand the condition to all Mondays.
Does 「祝日でも営業しています」 mean it closes on public holidays?
Show the answer and why
No. It includes public holidays among the days when it is open.