Triskaidekaphobia is a fear of the number 13, which is perfect for today! I’ll be including stories about a fear of any number, because triskaidekaphobia isn’t actually that common.
Books:

Thirteen Tales of Horror, edited by T. Pines, has all your favorite Point Horror authors in one collection. Thirteen authors, thirteen spooky stories… are you scared yet?

314 by A.R. Wise involves the fear of the number 314 (as well as the time, 3:14, and the date, March 14) and the strange events that happened in the town of Widowsfield years ago. This one is free on Kindle!

Five Midnights by Ann Dávila Cardinal begins with the death of one young man and the disappearance of another. When a second boy is found dead, his remaining friends begin to realize that the deaths have to do with their 18th birthdays, all in the month of July.
Movies:

Friday the 13th (1980) is of course on this list, even though the story doesn’t really have anything to with the number 13 or a fear of it. I’m not even sure they mention the date in the movie?

Thirteen Ghosts (2001) involves a haunted house where a countdown (count up?) of ghosts that become more and more dangerous as the numbers increase.

The Number 23 (2007) takes its title from a book in the movie. When Walter begins reading a book called “The Number 23” he becomes obsessed with the number.
Are you superstitious about the number 13?