'Вы всегда ходите в ботанический сад по воскресеньям?'
'Она всегда ходить в клуб в воскресеньи?'
One means 'on Sundays, the other means 'On Sunday'. Why choose on for one sentence, and the other for the other sentence, when (as I see it) the sense of both sentences are identical?
Thanks