It's National River Cleanup Day, so explore the terrors associated with pollution in The Toxic Avenger, Creepshow 2 and Godzilla vs. Hedorah. It's also the birthday of Raymond Burr, who starred in the Americanized version of 1956's Godzilla, King of the Monsters as well as 1985's Godzilla, so check those movies out as well. It's also the 57th anniversary of Godzilla Raids Again, so why not just make it a thing and throw in Godzilla vs. Megalon and Godzilla: Final Wars for good measure.
If lizards aren't your thing, National Learn to Swim Day affords you an excuse to watch Monster from the Ocean Floor, which also celebrates an anniversary today, along with Jaws, Deep Blue Sea, Sharktopus vs Whalewolf, the aforementioned Creepshow 2 and, for the lulz, Piranha 3DD.
It's also Rapture Party Day, so revisit your favorite religious horrors, from The Omen III to The Exorcist to The Prophecy.