Questions about Dates and Time:
Q: Why is it Lo hice el lunes and NOT Lo hice sobre/en el lunes?
Never use sobre or en before days of the week (even though English uses "on").
Q: Why is it viernes el 5 de diciembre and NOT viernes el 5, deciembre?
Do not use commas when writing dates in Spanish. Also note the spelling of "December" is with an "i" not an "e" in Spanish diciembre.
Q: Why is it Él llegó el sábado and NOT Él llegó el Sabado?
Names of days (and months) begin with a lower case letter unless they are at the very beginning of a sentence. Also remember that sábado has an accent.
Q: Why is it es la una and not son las una?
When the time is 1 o'clock, the verb ser and the article la should both be in the singular form, since there is only one hour you are counting. All other hours will occur with the plural verb form and article.
Q: Why is it son las siete menos cuarto and NOT son las siete menos cuatro?
Although both sentences are possible, the first means 6:45 and the second means 6:56. Be sure you don't use the word cuatro ("four") when you mean to say cuarto ("quarter"), as they differ only by the order of two letters.