Meanings
on Tuesdays, every Tuesday
Er ruft mich dienstags oft an.He often calls me on Tuesdays.
colloquialon (the next or last) Tuesday
Ich ruf dich dienstags mal an.I'll give you a call on Tuesday.
Examples
GermanEnglish
Ich sehe ihn nur dienstags in den Physikvorlesungen.I see him only on Tuesdays during Physics lectures.
Mathematik wird dienstags unterrichtet.Maths is taught on Tuesdays.