The measurement of time is different in Vedic Era. Weekday was not the part of that measurement. It is a subsequent development.
In Ramayana, there was no mention of vAra (weekday), but muhUrta, thithi, nakshatra, rAshi, etc, were mentioned.
In his Research paper Scientific Knowledge in the Vedas, Padmakar Vishnu Vartak says as follows:
The system of ...
Vara is intrinsic part of Jyotisha, hence Vedas, hence exist for all time in Sanatana Dharma.
Bhanu vasara - Sunday
Soma - Monday
Mangal - Tuesday
Budh - Wednesday
Guru - Thursday
Shukra - Friday
Shani - Saturday
The weekdays are related to the NavaGruhas. Pls don't ask for source. This is almost basic common sense knowledge.