Whenever I go there they seem to be out.
I read whenever I can, specially at night. (Preliminary English Test; B1; Italian)
I try to use olive oil whenever possible.
My friends have always regarded me as a cheerful, active girl, who is glad to help out whenever possible. (First Certificate in English; B2; Chinese)
"Will it be okay if I do it tomorrow?" "Sure, whenever."
Do it in a spare moment at the weekend or whenever - it really doesn't matter.