Just before the referendum. The lead at the time the referendum was called was much wider. One poll even had 66% remain in June 2015, while the Referendum Bill was in parliament.
Well, I just assumed that when you said "polls before the Brexit referendum" that you mean polls that were made before the referendum, and not when the referendum was called.
As opposed to a 20-ish% lead for Rejoin at the moment: https://en.wikipedia.org/wiki/Potential_re-accession_of_the_...