Why daylight changes with the seasons
Day length depends on two things: how far you are from the equator, and which way Earth’s tilted axis is leaning. The seasons are that lean, swinging back and forth over the year.
The next turning points
UTC dates. In time zones far east or west of Greenwich the moment can fall on the day before or after.
Tilt, not distance
Earth is slightly closer to the sun in early January than in July, but that isn’t what makes seasons. Earth’s axis is tilted about 23.4°, and it keeps pointing the same way as Earth orbits. For half the year the Northern Hemisphere leans toward the sun, and for the other half the Southern Hemisphere does.
Leaning toward the sun means the sun climbs higher and spends longer above the horizon: long days. Leaning away means short days. At the equator the lean hardly matters, so days stay close to 12 hours. Near the poles it dominates, all the way to midnight sun and polar night.
Opposite hemispheres, opposite seasons
That’s why, in late October, Buenos Aires is heading into summer with days of more than 13 hours while Oslo is losing several minutes of daylight a day. It’s the core idea behind the daily Daylite puzzle.