Mahashivratri festival is occured in chaturdasi tithi. Tithi is occured according to the moon. Actually it is occured for whole day that means for 24hours or 4prahar. 1 prahar means 6hours. It is a very special day for all persons. Young girls do this for better husband in future and for Virtue. Young boys do this for better wife and for virtue. All men and women do this for 4 things. They are Dharma, Artha, Kama and Mokhya and also for good health of their family. All people wish to free from sin.
According to karma good work give us good fruit but bad work give us bad fruit.
There are many stories for the mahashivratri.
one day a tribe do this pooja bidhi unknowingly. He awake whole night and bel leaves put on linga. But he gets the good fruit for this. So, devotees awake whole night and do pooja. Devotees do panchamrit snan on linga. Panchamrit means milk, curd, ghee, honey and sugar.
Another story is- Lord Shiva and godess parvati are married in this tithi.