Short answers, pulled from the story.
The seasons result mainly from Earth's axis of rotation being tilted about 23.4 degrees relative to its orbital plane, a tilt also called the obliquity of the ecliptic. Combined with axial parallelism, which keeps the axis pointed toward Polaris year-round, this tilt accounts for most of the seasonal variation in climate in both hemispheres.
The hemispheres always experience opposite seasons because only one part of the planet tips toward the Sun at a time. For about half the year, from around the 20th of March to around the 22nd of September, the Northern Hemisphere tips toward the Sun, and for the other half the Southern Hemisphere does.
Because of seasonal lag, the slow absorption and release of heat by the oceans, the heights of summer and winter occur up to seven weeks after the solstices. As a result, June, July, and August are the warmest months in the Northern Hemisphere, while December, January, and February are the warmest in the Southern Hemisphere.
The number of seasons varies by culture, from two to eight. Temperate regions commonly recognize four, the Hindu, Bengali, Odia, and Tamil calendars of India use six Ritu, Thailand recognizes three, and some Australian tribes and the Sami people of Scandinavia count up to eight.
Meteorological seasons group three whole Gregorian months, with northern spring beginning on the 1st of March, summer on the 1st of June, autumn on the 1st of September, and winter on the 1st of December. Astronomical seasons instead begin at the solstices and equinoxes, with the most common dates being the 20th of March, the 21st of June, the 22nd or the 23rd of September, and the 21st of December.
Seasonal weather and climate conditions strongly shape military operations, since firm ground from summer dryness or frozen winter snow aids manoeuvre while spring thaws and autumn rains inhibit it. Pre-modern European armies campaigned in summer because conscripts left at harvest, and navies depend on access to ice-free or warm-water ports such as Baltiysk, Vladivostok, and Sevastopol.