weather is complex but a couple items that should be mentioned:

heat is added all day and removed at night. The net flux of energy is that solar radiation adds energy during the day and black body cooling sends energy away from earth at night.

In summer, the solar radiation is higher (that is why it is summer) so there is more gain during the day. At night, a warmer black body can emit more energy so the dark planet can lose more energy at night.

Cloud cover dampens both effects so cloudy winters make this more pronounced.