Why are days like "Sunday" or "Monday" not mentioned in the Bible?

Names of the days of the week - Wikipedia

Except for Hebrew, the first weekday is named after the Sun, and the second after the Moon.

Romans and Greeks named the other days after their major gods, Mars, Mercuri, Jupiter, Venus, and Saturn.

English names the other days after the Nordic gods Tiw/Tyr, Woden/Odin, Thunor/Thor, and Frige/Freya. It follows the Roman convention for Saturday though, rather than the Nordic name, which means "washing day".

French names directly follow the Roman, except for the first day, which comes from the Latin word "dominicus". That word means "Lord's", and as the Sun was the Roman's primary god, it is indirectly named after the Roman convention.

The Hebrew names simply mean "first day", "second day", etc. except for the seventh day, "Shabbat", which means "rest" or "cessation".

The Old Testament writers would of course have written using the Hebrew names.

The New Testament writers would have avoided using names that honour pagan gods, and so would have used the Hebrew numbering

If you look up the Hebrew name for Sunday you learn that it means the first day...Hebrew word for Monday means the second day, etc. Showing the position in the week. Shabbat does not mean the seventh day, but "rest" or "cessation".

Because the words "Sunday", "Monday, "Tuesday, &c., are names assigned to the days of the week in English, and came into use hundreds of years after the books of the Bible were written in Greek or Hebrew.

Further, it may be that the custom in Hebrew, and perhaps in Greek, was to refer to days not by name, but by position, as in the Easter story, "Early in the morning on the first day of the week ...", where today English speaking Christians might be inclined to say instead, "Early on Sunday morning ...".

The ancient Hebrew people simply numbered the days of the week except for Saturday which was known as the Sabbath.

In Hebrew, the days of the week are simply numbered, except for the 7th, which is the Sabbath (Shabbat, שַׁבָּת‎‎ ). - The Days of the Week

As for the Greco-Roman tradition:

Between the 1st and 3rd centuries the Roman Empire gradually replaced the eight-day Roman nundinal cycle with the seven-day week. Our earliest evidence for this new system is a Pompeiian graffito referring to the 6th February (viii idus Februarius) of the year AD 60 as dies solis ("Sunday") - Names of the days of the week

It could be noted that some Liturgical books in the Catholic Church still use a numbered system for the days of the week.

In the Judeo-Christian or Abrahamic tradition, the first day of the week is Sunday. Biblical Sabbath (corresponding to Saturday), when God rested from six-day Creation, made the day following Sabbath the first day of the week (corresponding to Sunday). Seventh-day Sabbaths were sanctified for celebration and rest. After the week was adopted in early Christianity, Sunday remained the first day of the week, but also gradually displaced Saturday as the day of celebration and rest, being considered the Lord's Day.

Saint Martin of Dumio (c. 520–580), Archbishop of Braga, decided not to call days by pagan gods and to use ecclesiastic terminology to designate them. While the custom of numbering the days of the week was mostly prevalent in the Eastern Church, Portuguese and Galician, due to Martin's influence, are the only Romance languages in which the names of the days come from numbers rather than planetary names. - Days numbered from Sunday (Wikipedia).

As for the Roman Rite itself:

In the Roman Rite liturgy, the term feria is used in Latin to denote days of the week other than Sunday and Saturday. Some use the term even in English, but the official translation of the Roman Missal uses "weekday" instead. Various reasons are given for the Latin terminology. The sixth lesson for December 31 in the pre-1969 Roman Breviary says that Pope Sylvester I ordered the continuance of the already existing custom "that the clergy, daily abstaining from earthly cares, would be free to serve God alone". Others believe that the Church simply Christianized a Jewish practice. The Jews frequently counted the days from their Sabbath, and so we find in the Gospels such expressions as una Sabbati and prima Sabbati, the first from the Sabbath. (Wikipedia).