A magical full Moon is set to rise on Friday the 13th. Well.... that is if you live in Western half of the Western Hemisphere (West side of the Americas). Those in Europe and the Eastern United States / Canada will technically be in September 14th when the Moon hits the 100% full mark (but we won't mind too much if you claim you saw a full Moon on Friday the 13th).
Ok, so what does this mean?
Full Moons are often seen as a time of maximum magical effect. This is pretty much true, some spells or rituals are better timed at new Moons, or when the Moon is waning (shrinking) or waxing (growing), but generally people prefer full moons to do spells or rituals.
You may have heard about charging magical items. One way to do so is to leave the item outside over night and let the moonlight and starlight "charge" the item. So when there is a full Moon this effect is magnified because it receives as much moonlight as possible.
Friday the 13th
The number 13 has magical properties in many civilizations, and not always in a bad context. For example, in France the number 13 was a prediction of a better future. But most of the time 13 is seen as an unlucky number.
In Christianity it is particularly bad. Judas, who was said to betray Jesus was often called the 13th disciple. The 13th chapter of Revelations is dedicated to the Anti-Christ. And years with 13 full Moons were especially hard on the monks who had to calculate religious holidays.
From ancient tarot cards which often had the 13th card as the Death card, to a movie franchise about a mass murderer, to modern computers that get hit by viruses which only appear on this one day, Friday the 13th is a date with a lot of spiritual and magical meaning.
Bringing it Together
Like with all magic, intent is one of the most powerful ingredients of your spell. You can cast a good fortune spell on a full Moon on a Friday the 13th following the French belief of the day being lucky, or use the date as a day to bring misfortune to a rival or enemy. The choice is yours!