Just an interesting first person anecdote: In the south of Brazil (close to Argentina) we normally have well defined seasons, but almost like clockwork we always had a famous "mini-summer of May", an unusually warm week in an otherwise cold mid-autumn. Well in the last decade the new concepts of "mini-summer of June" and now "mini-summer of July" started entering the popular parlance, as this abnormally warm weeks during a cold season are becoming more numerous and coming later in the season (late Autumn and even Winter).
Veranillo de San Juan has been a thing since forever[0], and it usually was at the end of June. However, people have taken to call any hot spell this way, even if it's nowhere near St. John's date (June 24th). My anecdotal observations are similar to yours.
