Why is Mahashivratri called as Mahashivratri (night of Shiva) and not Mahashivadiwas (day of Shiva)?
Yes, I have spoken about it. Read the book. Raatri means that which gives you comfort, gives you rest; from what? That which gives us rest from the three Taapa (miseries) is Raatri: Aadi bhautika (Physical), Aadhyaatmika (Spiritual) and Aadi Daivika (Ethereal).
Taapayitra refers to problem/suffering; that time which takes us from misery towards relief is known as Raatri.