Super Squirt is live for US/OCE realms today, the 15th of September (16th for Oceanic realms)! Super Squirt is a rare occurrence where the Pet Battle Event lines up with Squirt being active in the Garrison Menagerie. It is very rare for these two to coincide, usually only happening a handful of times a year.
Squirt, or 'Super Squirt' as it's known when the events coincide, is important for pet battlers for a few reasons:
Patch 8.1, most Pet Battle Trainers in old continents were also made repeatable, at a reduced rate of experience. Squirt survived this experience nerf, and some other trainers did too. Family Familiar trainers (except Durian Strongfruit) still give a normal amount of experience, and can be repeated. Some of these trainers require a little more effort or RNG than Squirt, and are a lot less convenient, but are an alternative during Pet Battle Week for consistencies sake.
Notably, Sir Galveston is arguably the fastest of these trainers to repeat, however you will need to bring bandages.
Strategies for Squirt can be found in this guide, Squirt Pet Battle Leveling Guide, as well as in the comment section for Squirt.
Some of these guides may be useful for fighting Squirt!