What can come in groups of 7?

Answer 1

Some instances:

Days of the week,

Wonders of the ancient world,

Wonders of the modern world,

Pleiades (Seven sisters),

Ages of Man (Shakespeare, As You Like It),

Deadly sins (Christianity),

Notes in the Musical scale,

Colours of Rainbow (except that Newton, who originally studied the optical spectrum used only four colours - it is a continuum after all - but harmonising with musical frequencies (see above) led to seven colours.