Experienced Forks veterans will, of course, be familiar with the traditional algorithm. However, for the benefit of you Forks virgins out there, the traditional algorithm is as follows:

1. Find the date of the summer solstice (Hint: This is always June 21).
2. If the summer solstice happens to fall on a Saturday, that's the day. Skip all the remaining steps, because you already know when Plastic Forks will be held. Otherwise...
3. Note the dates of the two Saturdays closest to this date on either side.
4. Find the date of the full moon closest to June 21.
5. From the two Saturdays you noted in step two, select the one which is closest to the date of the full moon.
6. Arrive some time after four p.m., prepared to celebrate!