I might do a spell like this: Go to the bar and order a vodka soda.
Why? Well, 1, because it’s Friday and 2, Because credulous idiots have been acting all day like the confluence of the date on the Gregorian calendar with the day we call Friday has some significance with regards to luck/fate/magic/otherwise.
Seriously, nothing special about 13 Friday to witches, unless they work as fortune tellers or something, in which case they might see an uptick in business due to aforementioned credulous idiots.
Now, had you asked what spells are appropriate for Fridays in general (anything related to lust, fertility, sexuality and (odd, I know) children) or appropriate for the 13th day of the lunar cycle (usually a day before full moon, so great for anything involving increase and fruition.) then I have answers. But the calendar the Catholics made up has next to nothing to do with the Craft. It’s based purely on math and taxes, not on the actual cycles of Earth and its people.
Link to the original question on Quora.