How hot or cold is Mercury

In the day it's - 279 Fahrenheit (-173 Celsius) and at night it's 801 Fahrenheit (427 Celsius)

Well it all depends on the time of day, where it varies from very cold

(-279F or - 173C) at night to very hot (800F or 430C) during the day. This is due to the lack of atmosphere this planet has. And although it is the closest planet to the sun it is only the 2nd hottest planet