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 seven week-days was invented by the Indian sages at the
time of TaittirTya Samhita in about 8357 B.C. The source of this is
the Yajna system.
The Brahmins used to be 'Diksita' for six days and
then they were taking holiday on the seventh day. During this
holiday they used to give offerings only to the Sun, which was
supposed to be the Atman. So this day got the name of Aditya-Sunday.
On the other six days of the consecration, they used to give
oblations to the planets, one on each day, taking alternately from
the sun and from the earth.
He also stated as follows;
These week-days are mentioned in Atharva Jyotisa Sloka 93,
Yajnavalkya Smrti Acaradhyaya Sloka 295, Mahabharata Adi Parva 160/7
and Katyayana Grhya Sutras, third Kanda, fifth Kandika, second verse.
A special name 'Vara' is coined by Mahabharata and Katyayana for the
week days. Valmiki Ramayana also mentions Thursday as Brhaspati day