"On a Wednesday?"
Some languages use different prepositions in different contexts. Just because we use on wednesday and on wednesdays doesn't mean they use the same word for 'on' because english does. Though I'm just a learner like you, I've done various different languages and on the whole prepositions don't map exactly. I suspect if they were talking about the coming wednesday they would use il mercoledì
All that I've learned, when you say "We go on Monday", the right translation would be "Andiamo lunedì".
On other hand, when we state about some habitual activity, for example "I cook chicken on Monday / every Monday", the right translation is "Cucino il pollo il lunedì". When we want to say it in plural form, as in "We meet him on Mondays", it would be "Incontriamo lui i lunedì".
Please correct me if I'm wrong. :)
Though Duolingo is great in most ways, in such cases, the questions should be spaced such that we come across the Italian version before being asked to translate from English to Italian. How are we supposed to know instinctively that the 'on' part of this English phrase has no equivalent in the Italian translation?
Because that's 'the Wednesday' and while it can mean on Wednesday(s) that's not the same thing as On a Wednesday. On Wednesdays implies a regular occurrence. We go to the shops on Wednesday also implies a regularity. Here it's very specifically a singular occurrence. Imagine a conversation:
A: "I saw Antonio in town yesterday",
B: "On a Wednesday?"
C: "Yes, I know he's usually at home on Wednesdays"