。
On Thursday nights, the beer is free.
Literal
Thursday [possessive-の] night [topic-は] beer [subject-が] free is [explanatory-のです].
Classic 'X は Y が Z' double-subject structure: the time period 木曜の夜 is the topic (は), and beer is the subject (が) of 'is free.' This は + が pattern is one of Japanese's most distinctive: it lets a sentence have a topic that frames the comment without being its grammatical subject. のです gives the explanatory tone of someone informing you of a regular promotion.