Per day is 'par jour' in French.
'on this day' in french is 'ce jour-là'
par jour means by day / per day in French.
par
you say "jour" for day
To say, "Will you go out with me one day?" in French you say: Sortirez-vous avec moi un jour ?
Jour
Canada Day in french is "La fête du Canada"
today was a good day in french
On that day is "aujourd'hui" (for today) or "en ce jour" (for any day even in the past) in French.
C'etait une longue journee means "it was a long day" in French
It is "du jour".
You say "nouveau jour"... nouveau = new / jour = day.