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.

[0] It even has its own (poorly sourced) wikipedia article: https://es.wikipedia.org/wiki/Veranito_de_San_Juan