Why is the seventh birthday of the seventh lunar month not good?

Because the seventh day of the seventh lunar month is the day when the Cowherd and the Weaver Girl meet. They love each other very much, but they can only meet on this day. Although this day is a good day for lovers to meet, it also implies sadness, because the Cowherd and the Weaver Girl will be separated the next day, so this birthday is not good either, which indicates that there will be setbacks in the future marriage.