The Asylum blog compiles some solid advice on how to call in sick, even if you don't meet the traditional definition of "sick"—take that how you will. One of the best bits of advice involves the timing of your sick day, as noted by the editor of the Save the Assistants blog:
Pick a random Tuesday or Wednesday for your fake illness. At a lot of companies, there are mysterious sick waves on the day after a long weekend or on a really beautiful day in the summer. You can get away with that once in a while, but if you only come down with the flu on really beautiful Friday afternoons in the summer, everyone will be on to you. Taking off a day in the middle of the week will also make it less likely that you have 400 emails to sort through all at once when you come back from your elongated holiday.
Valid point, and the others are worth keeping in mind also. Photo by Perfecto Insecto.