mittwochs
on Wednesdays
Expresses that something regularly takes place every Wednesday or happens on a Wednesday.
Mittwochs gehe ich immer zum Yoga.
On Wednesdays I always go to yoga.

on Wednesdays
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on Wednesdays
Expresses that something regularly takes place every Wednesday or happens on a Wednesday.
Mittwochs gehe ich immer zum Yoga.
On Wednesdays I always go to yoga.

The adverb 'mittwochs' (on Wednesdays) is formed from the noun 'der Mittwoch' (Wednesday) by adding an '-s'. This '-s' at the end of time expressions always signals regular repetition.
Der Supermarkt hat mittwochs länger geöffnet.
The supermarket is open longer on Wednesdays.
Although it is derived from the noun 'der Mittwoch' (Wednesday), 'mittwochs' (on Wednesdays) is an adverb and is therefore always written in lowercase.
Wir treffen uns immer mittwochs zum Abendessen.
We always meet for dinner on Wednesdays.
As an adverb, 'mittwochs' (on Wednesdays) can stand in position 1; then the verb follows immediately. But it can also stand in the middle of the sentence after the verb and the subject.
Mittwochs hat mein Lieblingscafé geschlossen.
On Wednesdays my favorite café is closed.
Wednesday
'der Mittwoch' and 'mittwochs' both refer to the same day of the week. 'der Mittwoch' is a noun and means the day itself, often a specific Wednesday. 'mittwochs' is an adverb and usually means regularly on every Wednesday. The words are easy to confuse because they look almost the same. If you are talking about the day as a date or appointment, use 'der Mittwoch'. If something happens every week on that day, 'mittwochs' fits better.

Mittwochs arbeitet Lena immer bis sechs Uhr.
On Wednesdays, Lena always works until six o'clock.
Der Mittwoch passt mir gut für den Termin.
Wednesday works well for me for the appointment.