Learn French: Tu es libre mercredi ? - Are you free Wednesday?

Using le before a day of the week turns it into a recurring habit. If you said Tu es libre le mercredi ?, you would be asking 'Are you free on Wednesdays (in general)?'. Leaving out le means you are asking about this specific, upcoming Wednesday.

Why is there no word for 'on' before mercredi?

In English, we say 'on Wednesday,' but French doesn't use a preposition there. You simply state the day of the week by itself.

Why isn't mercredi capitalized?

Unlike in English, days of the week and months of the year are not capitalized in French unless they happen to be the very first word in a sentence.

This looks like a statement. How do we know it is a question?

In everyday spoken French, you can turn a regular statement into a yes/no question just by raising the pitch of your voice at the end, exactly like saying 'You are free Wednesday?' in English. You don't always need to add question words or swap the verb and subject.