When do the swallows fly away from San Juan Capistrano?

M: So, Rebecca, what did you do for spring break?
W: I went to southern California. I was in Los Angeles and San Diego. Oh, and I stopped in San Juan Capistrano. That’s south of Los Angeles and north of San Diego. I happened to be there just about the time the swallows returned.
M: Oh, I’ve heard about that—they always return on the same day, don’t they?
W: That’s right, on March 19th. And they always fly away on the same day, October 23rd. In the meantime, they migrate over 7,000 miles to get to their winter homes.
M: Seven thousand miles—imagine! And always coming back on the same day!
W: Yes, except for one year, a long time ago—they were delayed for a day by a storm at sea.
M: So, what’s the town of San Juan Capistrano like?
W: Oh, it’s a pleasant little town. Once there was a famous mission church there.
M: Once? What happened to it?
W: It was destroyed by an earthquake almost two hundred years ago. But there is an old adobe church that survived. The swallows build their nests in the walls and towers of that church.
M: You sure were lucky to be there on the one day of the year when the swallows return.
W: Well, I wasn’t there exactly on that day. I got to town a couple of days later—but I did see the parade celebrating the swallows’ return.
M: They have a parade? The people there must really like those swallows.
W: Sure—they bring lots of tourists to town, and besides, the swallows eat insects—including mosquitoes!