Short answers, pulled from the story.
The British Invasion began in late 1963 when American media started covering Beatlemania. The phenomenon was triggered by national newspaper articles about the Beatles frenzy appearing in October 1963 followed by their Royal Variety Performance on the 4th of November.
The Beatles achieved the first number one hit with I Want to Hold Your Hand which reached the top of the Hot 100 on the 1st of February 1964. This song had already vaulted to number one on Cash Box magazine charts by the 25th of January 1964.
On the 8th of May 1964, eight out of nine entries in the Top Ten were from British Commonwealth countries. That same week only Gary Lewis & the Playboys held a spot at number two instead of a British artist.
British fashion and film became major trends with Mary Poppins winning multiple Oscars and The Beatles influencing youth clothing styles. Television series like Danger Man and The Avengers also gained popularity among American viewers starting in the mid-1960s.
The British Invasion ended around March 1969 when The Guess Who achieved success with These Eyes marking the start of a Canadian Invasion. British musical acts retained their popularity into the 1970s while new waves emerged later including power pop and the Second British Invasion.