The names of the Week Days are derived from where ?
Why Monday is after Sunday?
Why Tuesday is after Monday ?

The answer is given by Astrology as Planetary Hours or Kala Horas. If you
have any other explanation - from Astronomy or any other sciences - pls let us
know

Question - How did Monday come after Sunday in the Calendar ?

Answer - To answer this question, we have to go to the concept of Planetary
Hours or Kala Horas

The Planetary Hours are counted from Sunrise and the owner of the Planetary Hour is the Lord of the Weekday. Today is Sunday and one hour from Sunrise is presided by the lord of Sunday, the Sun.

The sequential order of Planetary Hours is as follows.

Sun
Venus
Mercury
Moon
Saturn
Jupiter
Mars

( Arka Shukra Budhas Chandra Manda Jeeva Dharasutha, Kramena Kala
Horesa.... )

So if the first Planetary Hour on Sunday is presided by the Sun, the nexthour is presided by Venus, next by Mercury and so on. This goes on for 21 hours and the 22nd Planetary Hour is presided by Sun, the 23rd by Venus, the24th by Mercury and the 25th, the first Planetary Hour next day, is presided by the Moon and hence his day, Monday, became the next week day in the
Calendar.

We have these basic equations

Sunset - Sunrise = Duration of Day, Dinamana
Sunrise (next day ) - Sunset = Duration of Night, Ratrimana

The diurnal duration is divided by 12 to get Planetary Hours. The diurnal
duration will vary, as Day = Night = 12 Hours occurs only on Equinox days.
Similarly the nocturnal duration is divided by 12.

Dina Dwadasamso Matha Kala Hora
Pathi Thasya Poorvasya Varadhinadha
Thadha Shashta Shashta Kramena Tharesam
Nishyayam thu vareshwarad Panchamadya.

Here is a table depicting the Weekday Numbers and Planetary Hour Numbers

WeekNo Planet PHNo
1 SUN 1
2 MOON 4
3 MARS 7
4 MERCURY 3
5 JUPITER 6
6 VENUS 2
7 SATURN 5