Why is 7 an auspicious number?

Seventh, the auspicious number is generally for girls. In ancient times, seven represented exquisiteness and was very popular with girls' relatives. Generally speaking, the number seven on a girl's birthday is considered to be particularly meaningful. Moreover, in many idioms, the ancients' love for seven is also very obvious, such as seven up and eight down, seven open and eight down, seven steps and eight forks and so on. But as far as modern people's thinking is concerned, seven is gradually not included in auspicious numbers, but only occasionally appears in some constellations.