Questions about Date and Time
Q: Why is it Je l'ai fait lundi and NOT Je l'ai fait sur lundi?
Never use sur before days of the week (even though English uses "on"). Either write nothing, or write le if you mean "every".
Q: Why is it le vendredi 5 décembre and NOT le vendredi, 5 décembre?
Do not use commas when writing dates in French.
Q: Why is it Il arrive samedi soir and NOT Il arrive Samedi soir?
Names of days (and months) begin with a lower case letter unless they are at the very beginning of a sentence.
Q: Why is it Le premier janvier and NOT Janvier premier?
To indicate the first, second, etc. of the month, use the French order, not the English one.