I have learned this from listening to katha of Giani Thunka Singh Ji and from the time I spent at budha dal.
There is this sakhi about reading chandi di vaar at night.
Baba Ji would also do Chandi Di Vaar in his Nitnem every day. But a
question arose in his mind. When should this Baani be recited and
how often? He would often wonder. He asked many religious and
saintly people but nobody really gave a firm answer. One day, while
in the hut, he began reciting Rahiraas Sahib after which he did
Chandi Di Vaar. He then finished off with Sohilaa and was preparing
to go to sleep. The moon was shining outside and the door to the
hut was open. He had just lay down when all of a sudden, he saw
two men approaching from a distance. Baba ji thought to himself that
if they are travellers then he will provide them with food and
clothes and offer them a place to rest for the night. But if they
are robbers, he will be prepared to confront them. While he was
still thinking they both pinned him to the ground.
One held him down from the head and the other from the feet. They
couldn't be travellers or robbers because they arrived at such
incredible speed. They were Shaheed (martyred) Singhs. He began
doing Mool Mantr Jaap. He had just completed two Mool Mantrs when
the Singhs released him. He sat up. The Shaheed Singhs explained to
him that they didn't come to hurt him.. They came to remove his
doubt and confusion about the recitation of Chandi Di Vaar and when to
do it, how much to do and how to do it. It was the fact that he
thought that he would confront them that they wrestled him to the ground.
They explained that if Chandi Di Vaar is recited after the sunset,
then it must be continued all night non-stop. It is alright for two
or more Singhs to do it in turn. If an individual can recite it all
night then that is alright as well. A clean cloth should be spread
underneath where you're sitting and you must have your bath first.
An oil lamp (dhesi ghior dee joth) should be lit.
There are no strict rules about reciting in the day time. Whether it
is recited one or more times, it should be done with love and
affection. After saying this, they left the hut and then suddenly
disappeared.