Hamlet Why must the ghost leave during day time? Act 1

The ghost much leave during day time because ghosts, in purgatory, were not allowed to walk during the day which was sacred. Marcellus explains:

Some say that ever 'gainst that season comes Wherein our Saviour’s birth is celebrated, The bird of dawning singeth all night long. And then, they say, no spirit dare stir abroad. The nights are wholesome. Then no planets strike, No fairy takes, nor witch hath power to charm, So hallowed and so gracious is that time.