FYI: credit cards are valid through the last day of the month they expire in. A card expiring in YYMM == '2006' can still be used today.
You will note that almost every source on the matter says that this is "generally" true, or true for "almost all" cards. The specific day of the month is not a requirement baked into any formal credit card specification, and most finance sites instruct you to contact your issuer if it matters to you.
Cards that expire at the beginning of the month used to be more common, but with Visa and Mastercard dominating the credit card market, other providers have had a tendency to follow their lead to avoid surprising their customers.
Still: Having spent far too long dealing with ecommerce checkout pages in the past, I advise against making expiration date validation a hard error. If you want to caution the user if it looks like it might be expired, that's up to you, but let them submit it anyway!