Pascual Segura has researched the topic and found this:
It was in the year 1584 that San Joaquín first had his own date on the calendar: March 20th.
Then, in 1738, Pope Pius X moved the date to August 16th.
It was in 1969 that Pope Paul VI decided that the feast of San Joaquin should be joined to that of Santa Ana and that both would be celebrated on July 26th.
Despite this change of dates, Bigastro, like other municipalities with which it shares its patron, still celebrates San Joaquin on August 16th. It is a tradition with more than two centuries of history.