Resources: Astrology : Panchangam Calculations by Karanam Ramakumar

Very clear, useful and informative.

You can understand how the longitudes of ravi and candra can be used to calculate tithi, nakSatra, yoga, karaNa, muhurta, amRta ghadiya and varjyam(tyajyam) in the nirAyana (sidereal method).

durmuhurta, gulika, yamaganDa, rAhukAla etc. are based on the day of the week.

[Unrelated note : it is said that the the days of the week are really two days or so off. So for selecting a good day we can’t rely totally on vAra.]

It also becomes clear why the time just before sunrise is so magical.