Mr. Popular received 366 greeting cards in 1987. Are there any cards sent on the same day? (1987 is not a leap year.)
The year 1987 or any non-leap year, for that matter has 365 days. Imagine putting 366 pigeons in 365 holes. We can see that we must put more than one pigeon into at least one pigeonhole, so there must be a day on which more than one card is sent.