It is considered to be lucky. In Christianity, 7 symbolises spiritual perfection. 3 is a special number also, because of the Trinity, so 7 is repeated three times.
Thats only from a spiritual perspective.
But it's really not that lucky, dates are manmade anyway.
It isn't. Some people are probably going to be involved in traffic accidents, injuries, homicides and other assorted mayhem.
Seven is considered lucky because it was the number of the visible planets in the night sky and that is where the ancients believed the gods lived, so if you had all seven of the gods on your side, that was "lucky".