Why doesn't esperanto have a nice, orderly system like chinese or hebrew? i.e. tago unua, tago dua, ktp. or even unutago, dutago, tritago, ktp. if you don't want it to conflict with e.g. "first day". That seems more in line with esperanto's philosophy of keeping things simple. The only reason I can think of is that it would be arbitrary to decide which day to start on (chinese and hebrew differ on this), but frankly, I don't think it matters as much as having a simple system to begin with.
I think this would introduce more problems than it would solve. It's not only Chinese and Hebrew where a difference exists over which day of the week is "Day One". One website I looked at, https://www-01.ibm.com/software/globalization/guidelines/c11.html says that different parts of the world, Saturday, Sunday or Monday are considered the first day of the week. (I don't know how accurate that site is, except that other sites give similar information).
Also, as Esperanto has now been in existence for 130 years, it would seem a bit late to change something as basic as the names of the days of the week. After all, even if that happened, there would still be 130 years' worth of Esperanto literature in which dimanĉo, lundo, mardo etc. are used, so those names would still need to be learned.
it probably has to do with religion (except for chinese I guess). For christians it starts on monday because sunday is the holly day of the week, hence the last one, when God finished to make the world. But that holly day is saturday for jewish people and friday for muslims...
I haven't heard of Christians saying that Monday is the first day of the week. Most Christians would say, as the bible does, that Sunday is the first day. Jesus rose from the dead on the first day of the week, which is why most Christians have their services on Sundays.
Why were the days of the week basically copied form Latin/ romance languages. the names come from the celestial bodies seen from earth and their associated god(esse)s [ Monday= moon day, thursday= thor's day (Germanic equ. of god of thunder Jupiter), dydd Llun= luna/moon day, mardi= mars day, (demanche and other romance equv. Sunday is named 'the lord's day' from dominica = lord; and Saturday named after judo-christian Sabbath).
Surely it's be easier to use the Esperanto word for Moon [Luno], Mars [Marso], Mercury [Merkuro], Jupiter [Jupitero], Venus [Venuso], Saturn [Saturno], Sun [Suno] + the Esperanto word for day [tago] (luntago, marstago, murkutago, jupitago, venustago, saturntago, suntago). which would also be less confusing than numbering the day and more true to the etymology and consistency of Esperanto.
Because it's borrowed from French semaine which has an -i in it.
The Esperanto root is semajn- and the presence of a -j and an -n at the end is completely unrelated to the plural or accusative case endings.
A bit like how in English, "census" always has an -s at the end even if you're just talking about one of them; it's never spelled "censu" because the -s at the end is not the plural marker but part of the root.
The thing with Esperanto is that it uses a lot of endings and other affixes to indicate parts of speech (plurals, adjectives, bad things, fractions, multiples, verb tense, adverbs, etc.). But there are many Esperanto words that contain letters used in those endings as part of the root of the word. For example, the affix "-in" makes a word refer specifically to a female (kelnero = a waiter; kelnerino=a waitress), but the word for child is "infano" - it cintains the letters -in, but not as an affix. Similarly, by no means is every Esperanto word containing the letter j necessarily plural. We'd be very restricted if letters used in affixes couldn't be used anywhere else!