Why does the di come before for Sunday in french?

Answer 1

For etymological and religious reasons.

Dimanche comes from the Latin "Dies dominicus" (Day of the Lord) and have become diominicu, due to what linguists call a consonantal dissimulation. Dies dominicus was itself the translation of Lord's day in Greek "Kuriake Heméra" into christian Latin.

While Sunday comes from the Roman Latin Solis Dies (day of the sun).

It seems, in the early Middle Ages, the south of Europe used the "Lord's day version" version while the north had used the Sun's day version. Perhaps because Christiany imposing the Lord's day rather the Sun's Day had not "invaded" the Pagans culture yet.

While the other days names remained influenced by the names of roman gods. The sixth and seventh days had become differents from northern countries keeping the original latin basis to the south of Europe, highly christianised. Thus, samedi in French comes from Sambati dies (sabbath's day) while saturday comes from Saturni dies (Saturn's day).

Answer 2

In French, the preposition "di" is used to indicate "on" when referring to a specific day of the week. It is a convention in the French language that has developed over time to differentiate the phrase from a general reference to a day.