As you may know, in Vietnamese the names of the days of the week are
If we translate them litterally into English, we’d have “second [day]”, “third”, “fourth” and so on, and lastly “day of the Lord” (the old name for Sunday in Vietnamese is Chúa-nhật, which in fact means “day of the Lord”).
Why are they so different from, say, the French language days of the week, despite 100 years of French colonization? A conjecture from the writer is… the Portuguese.
As a matter of historical fact, the Portuguese Catholic missionaries were some of the earliest linguists ever that set foot on Vietnan, around 16th or 17th century. Back then, the Vietnamese people wrote in Chinese or Nôm characters, the latter itself being an invention derived from the former. In order to facilitate the teaching of Gods, those missonaries learned the native language and began to transcribe it using their own writing system. Guess what, that was the birth of Quốc ngữ – Vietnamese language with a writing system based on Roman alphabet
The Portuguese missionary also applied how they called the days of a week. In their own language it goes like
domingo (Sunday, and etymologically means “day of the Lord”, back then considered the first day of the week),
The days with –feira then dropped the word in colloquial language, and became segunda, terça, quarta, quinta, sexta, meaning respectively “second”, “third”, “fourth”, “fifth”, “sixth”, and came into new-born Quốc ngữ as thứ hai, thứ ba, thứ tư, thứ năm, thứ sáu. There was no cost at all in inventing thứ bảy to replace the exotic sábado, and translating domingo into Chúa-nhật, which later became Chủ nhật. Note that before the French colonization, which began in the later half of 19th century, in Vietnam there was no similar naming system, except in some Christian parishes; people were calling days by the zodiacs. Later the colonizing authority may have adopted the Portuguese-based system to facilitate governance.
Note that thứ hai, thứ ba, thứ tư, thứ năm, thứ sáu, thứ bảy are also words for ordinal numbers. If we add ngày (“day”) into the names, ngày thứ hai means either Monday or the second day of a period.