Saying 'on Monday': days of the week and the definite article
English speakers naturally want to translate the word "on" when talking about days of the week. If you want to say that something happens on Monday, you might be tempted to look for a direct translation for that preposition. However, Italian handles the days of the week quite differently.
You will never need a preposition to say that an event happens on a certain day. Do not try to use words for "on" or "in". Instead, the meaning of the sentence changes entirely depending on whether you use the day of the week entirely on its own, like lunedì, or with a definite article, like il lunedì.
To talk about an event happening on one particular day, simply use the name of the day without any article or preposition. You just drop the English "on" and state the day.
This rule applies to both past and future events. A day of the week used on its own indicates a particular occurrence, and the verb tense will tell the listener whether that specific day has already passed or is yet to come. Let us look at lunedì and mercoledì used for specific events.
If you want to talk about a recurring habit, like doing something "on Tuesdays" or "every Friday", Italian adds the definite article before the day of the week. The article does the heavy lifting, showing that the action is a regular routine rather than a one-time event.
Six of the seven days of the week are masculine, so they take the singular masculine article il. You will see this pattern with lunedì, martedì, mercoledì, giovedì, venerdì, and sabato.
There is one exception to the masculine rule. The word for Sunday, domenica, is the only feminine day of the week in Italian. When you talk about a one-time event, you still use it without an article. However, when you describe a routine that happens on Sundays, you must use the feminine article la instead of il.
When talking about days of the week in Italian, leave out the preposition "on". Use the day without an article to talk about a single, specific event in the past or future. Add the definite article (il for Monday to Saturday, la for Sunday) to talk about a repeating habit or routine.