Answer:
During this time period, the axis of the earth is tilted by 23.5°. As our earth revolves around the sun and completes its one rotation in 365 days. During this rotation in the month of June the Northern Hemisphere “leans into” the sun and experiences more radiations coming from the sun.
That is why it is more illuminated and hot. While in December Northern Hemisphere leans away and the Southern Hemisphere leans into the Sun.
Whereas in September and march the earth the hemispheres are equally favored by sunshine and the earth neither leans into the sun nor away from the sun.