You’re not supposed to cite Wikipedia in your assignments, but hey, let’s stick it to the man. And there are occasions when citing Wikipedia is appropriate. Maybe you’re writing an assignment about Wikipedia!
For the year of publication, use the year when the Wikipedia page was last modified. This information appears at the bottom of the page:
This page was last modified on 17 August 2017, at 17:34.
Here is the format for a Harvard reference to a Wikipedia article about lithic flakes (archaeology).
(‘Title of article’, Year)
Channel flakes are caused by the fluting of particular Paleo-Indian projectile points ('Lithic flake', 2017).
‘Title of article’ (Year) Wikipedia. Available at: https://en.wikipedia.org/wiki/… (Accessed: date).
'Lithic flake' (2017) Wikipedia. Available at: https://en.wikipedia.org/wiki/Lithic_flake (Accessed: 26 June 2019).
The same format is used when citing other wikis:
'Caxambu Style Borborygmus Potion' (2018) Harry Potter Wiki. Available at: https://harrypotter.fandom.com/wiki/Leaky_Cauldron (Accessed: 27 June 2019).
'Corybas limpidus' (2018) Wikispecies. Available at: https://species.wikimedia.org/wiki/Corybas_limpidus (Accessed: 27 June 2019).
'Law of Cosines/Proof 3' (2016) Proof Wiki. Available at: https://proofwiki.org/wiki/Law_of_Cosines/Proof_3 (Accessed: 27 June 2019).