The 7th month of the year was special in Israel just like the7th day of the week and the 7th year was special. It began with great fanfare during the Feast of Trumpets (Numbers 29:1-6). Now, the 7 feasts of Israel were just that - feasts; celebrations; festive occasions... with one exception. On the Day of Atonement the people were to be intentionally sad. On the 10th day of the 10th month they were supposed to "afflict their souls" (Numbers 29:7) because of their sinfulness. Even the Passover was to have been celebratory, but this day, when the sin sacrifice was the focus of the nation, joy was to be set aside.
Think of the crucifixion of our Lord. On the great day of final atoning, Jesus labored and sacrificed miserably as His Father and all of nature recoiled in agony at the scene... there was nothing to smile about. Sin and all of the awfulness of it was on display. It would not have been an appropriate context for joy or celebration. We celebrate His resurrection, yes... but we should bow our head in somber shame when we think of how inhumane humanity was in our handling of the Messiah (see Isaiah 53).