A variety of studies have come up with some interesting information on the peak environmental factors that affect you gettin' it on. Check the via link for the full list, but here are some of the highlights:
December has the highest number of conceptions, and August the lowest.
More than twice as many condoms are sold the week before Christmas than the week after.
Sex is 17x more likely to happen at midnight than at 10 a.m.
Sunday is the least popular sexy times day (Friday and Saturday at the most popular).
Sex is far more likely to be rejected due to a room being too hot than too cold.
4 out of 5 Americans say the best time for sex is when it's raining outside.
So in conclusion: if you try to do the horizontal boogie on a Friday or Saturday, in December (the week leading up to Christmas, specifically), in a room that's about 62°, while it's raining, you're pretty much good to go.