Paraphilia Phriday 7
Someone who has sex only on Friday the 13th or fantasizes only about having sex on Friday the 13th is a friggatriskadekaphiliac.
Friday is named after the Norse goddess Frigga, a.k.a. Freya.
If you want to stick with Greek, you can call it paraskevidekatriaphilia, but the Norse version is quite apropos in this context.