What effect could the length of days on Venus have on the temperature of its surface?

The extraordinarily slow rotation of Venus (once every 243 Earth days) could have the effect of increasing the difference between temperatures on the "day" side as opposed to the "night" side, as occurs on Mercury. A day or night on Venus is 58 Earth days long. But this temperature variation does not occur due to its dense atmosphere.

Venus has an incredibly thick atmosphere that doesn't let most heat escape. It has a lot of carbon dioxide in its atmosphere (a greenhouse gas) which traps heat energy re-radiated by the planet. The atmosphere also effectively tranports this heat around, so it is also practically as hot at "nighttime" or at the poles.