Question

why the temperature of mercury is very hot in day and very cold in night

Answer 1

Because Mercury has an incredibly tenuous, basically non-existent atmosphere to retain any of the heat that the day side can absorb. Because Mercury has no medium for heat to spread in, its nights get frigid. All the heat that is transferred to the night side is rapidly lost from infared radiation. Also because Mercury’s day/night cycle are very long, giving the night side a lot of time to cool down. We can see the extreme heat differences on some of the craters near the poles on Mercury, the bottoms of some are permanently shadowed leaving them at very low temperatures, perpetually.

Answer 2

it happens because Mercury logest all the heat for the warmth there so, it is hot in day.