What date was the circuit, snetterton, when riki christodoulou had the fastest lap?
CREATE TABLE "calendar" (
  "circuit" text,
  "date" text,
  "pole_position" text,
  "fastest_lap" text,
  "winning_driver" text,
  "winning_team" text
);
SELECT "date" FROM "calendar" WHERE "circuit"='snetterton' AND "fastest_lap"='riki christodoulou';