Could someone please explain solstice?!

Could someone please explain solstice?

Was reading my horoscope and it said : On Thursday, the Sun enters your sign of Cancer, and along with marking the start of the Solstice, it is also the beginning of a month-long celebration for you.

Summer is a season of the year that is defined as beginning in June, and ending in September in the Northern Hemisphere. In the Southern Hemisphere, summer begins in December and ends in March. Summer is defined by convention in meteorology as the whole months of June, July, and August, in the Northern Hemisphere, and the whole months of December, January, and February, in the Southern Hemisphere. The unofficial start of summer is a matter of convention: in Ireland it is as early as May 1, in many countries it is considered to be June 1, while in others it is as late as July 1. In general, seasonal changes occur later in coastal regions, so countries close to the oceans go for an later start to summer than inland ones. Summer is commonly viewed as the season with the longest (and warmest) days of the year, in which the daylight predominates, through varying degrees Hopes this helps.