Gurudev, Monday is for Lord Shiva and Friday is believed to be for Goddess (Devi). What is the relation between time and God?
All the days are for you and me only (laughter). All the times are for us only.
It is a belief that days are associated with planets (Graha), and planets belong to one or the other God (Devta). But you should not worry about these things too much.
'Shivaratri' does not come only on Mondays, it can fall on any other day of the week. The day on which 'Shivaratri' falls, that day is of Lord Shiva only. It is not that only Monday is for Lord Shiva. Lord Shiva has control over some stars and planets, hence a rule is made like this. Not only days, but even Nakshatras have respective ruling deity. If you want to pray then pray to Paramatma (God), all the Gods are inside Him.
That is why there are ‘Ishta-devta’ (Deity of entire family). We have Kul-devta and Ishta-devta for families, then Gram-devta for an entire village, Kshetra-paal for a particular area. All these rules are fascinating.
We have prayers for three levels
1. Adhyatmik: where we have knowledge of self. We also have Adhyatmik traditions, etc.
2. Adhibhautik
3. Adhidaivik
That is why we say, ‘Shanti(Peace)' three times, for Adhidaivik, Adhibhautik, Adhyatmik.
We should also keep in mind that we are not keeping fast for God. If you fast on a Friday, then Goddess will become happy. If you fast on a Monday, Lord Shiva will get pleased, nothing like that! You are not fasting on Monday to please Lord Shiva. You are fasting just for your own sake, for your health, for body cleanliness and purification.