AFAIK, the subjunctive should be used in the sentence "They wish it were June", but I have seen "They wish it was June" in this blog: https://blog.abaenglish.com/intermediate-grammar-how-to-use-wish/ . Am I mistaken about using the subjunctive here, is the sentence in the blog wrong or may the indicative replace the subjunctive?
You are absolutely right. This is a textbook example of a case in which the subjunctive mood should be used, so "were" is correct – at least, according to the traditional rules of English grammar. Nowadays, many native speakers do use "was" instead of "were" in sentences like this, presumably because they think "were" sounds odd or wrong with a singular subject. Still, it's jarring to see "They wish it was June" presented as "correct" by someone who is offering to teach English to others.
I'd look elsewhere for my English lessons if I were you!