。
June is the social season in London.
Literal
June [topic-は] London [in-で] [topic-は] socializing [possessive-の] season [copula-だ].
Double は: one topicalizes 六月, the other topicalizes ロンドンで. 社交 (しゃこう, 'socializing') refers to the British tradition of the London Season of high-society events. Cultural knowledge needed for full appreciation.