noun, noun which may take the genitive case particle 'no'
no advance reservation required; walk-in welcome
Used in signs, advertisements, or descriptions of services that do not require booking ahead of time.
You can join this event without an advance reservation.
We also offer medical consultations that do not require an appointment.
予約不要 means 'no reservation needed' in general, while 事前予約不要 explicitly emphasizes that advance booking is not required.
当日受付 means 'same-day registration' and is often used alongside 事前予約不要 to indicate walk-in availability.
Compound of 事前 (beforehand), 予約 (reservation), and 不要 (unnecessary).