A little known fact about a coastal Southern California Slug (Deroceras invadens) is it undertakes a migration to over winter each year near Mission San Juan Capistrano. Scientist have been studying this phenomena for decades. One theory focuses on the interaction of the Swallows and the slugs. After the Swallows migrate from Mission San Juan Capistrano for the winter, the area is a ecological safe zone without the slugs major predator. Another theory focuses on the long history of cultivated ground in the area with all the compost that is so perfect for the slug’s life cycle.

But each year, just before the Swallows return, the slugs leave San Juan Capistrano and return to their native home ranges, thus supporting the first theory. And each year as the slug migration begins, slug watchers gather in the early morning hours for the annual slug count. And each year the Southern California gardeners dread this date.

So celebrate today, get outside, count some slugs; but please forgo the Escargot for lunch.

Yes this is a real registered holiday. Here is the whole story.

This silly holiday is a parody of the holiday called “Swallows Return to Capistrano.” Swallows Day is supposed to be a lovely day in March when flocks of cliff swallows arrive at the beautiful Mission San Juan Capistrano and begin to build their springtime nests. For years people have had a parade and festival to welcome back the sparrows, and they traditionally ring the mission bells as well. Unfortunately, there have been very few or no swallows anywhere near Capistrano, recently. And I don't mean few or no swallows on Swallows Day; I mean few or no swallows anywhere near Capistrano at all, ever.[1]

Still, from what I can tell, Swallows Day goes on, with or without the swallows![1]

Today's “holiday,” Slugs Day, warns people all over the nation that the slugs are returning from their winter headquarters in Capistrano—but of course, that's a great big lie! Slugs do not migrate each year, and they certainly don't overwinter in Capistrano![1]

There really is no slug migration, just like there is no swallow migration specifically to Mission San Juan Capistrano.

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[1] http://every-day-is-special.blogspo.....turn-from.html

http://etsyforanimals.blogspot.com/.....trano-day.html
Some interesting other days as well.

http://www.wellcat.com/may/slugs_re.....strano_day.htm

http://www.allaboutslugs.com/faq/

http://ucanr.edu/sites/CalSnailsand.....ve_field_slug/