How Hindu Weekdays got their names?

Ravivar/Aadivaaramu, Somavaar, Mangalvaar, Budhavaar, Guruvar, Sukravaar, Sanivaar.

These are the names of Hindu Weekdays.

Rishis divided time into what they call Hora kaalam. Each Hora kaalam stays for about an hour.

There are 7 horas,

1. Surya Hora
2. Shukra Hora
3. Budha Hora
4. Chandra Hora
5. Sani Hora
6. Guru Hora
7. Kuja Hora

These 7 horas repeat cyclically in that above order, every one hour.

So in a day, we will have 24 horas. First Hora starts with Surya. Another name of Surya is Ravi. So we call it Ravivar or Aadivaaramu.

If you see above, Ravivaar ends with Budha Hora. The 25th Hora is Chandra Hora when next day starts. Another name of Chandra is Somu. So we call it Somavaar.

Now, if you count cyclically again for Somavaar, it ends with Guru Hora. Next day starts with Kuja Hora. Another name of Kuja is Mangala. So we call it Mangalvaar.

Mangalvaar ends with Shukra Hora as it is the 24th hour. And next Hora is Budha Hora which is why we got Budhavaar.

This is how our Rishis gave the name for weekdays.

Source: BrahmaSri Chaganti Pravachanam