有効 and 無効: a valid ticket and one without validity
Read validity periods and eligible weekdays, and distinguish ticket validity from the effectiveness of a method.
Valid under specified conditions
有効, read ゆうこう, means valid or usable. この券は今月末まで有効です。 says that the ticket is valid through the end of this month.
この割引券は平日のみ有効です。 says that the discount coupon is valid only on weekdays. のみ limits the eligible days, so do not extend use to weekends.
Invalid or without effect
無効, read むこう, means invalid or without effect. 有効期間を過ぎた券は無効です。 says that tickets past their validity period are invalid.
入力した番号は無効です。 indicates that the entered number is invalid. It does not say whether the reason concerns its length, validity period, or another condition.
Ticket validity and a method's effectiveness
この方法は時間の節約に有効です。 says that this method is effective for saving time. Here 有効 has a different contextual meaning from a usable ticket.
A valid ticket may still be restricted by location, date, or eligibility. 指定の店舗でのみ有効です。 says that it is valid only at designated shops, not at other shops.
Can a coupon described as この割引券は平日のみ有効です。 be used on weekends?
Show the answer and why
Not under this notice. 平日のみ limits use to weekdays.