What is the earliest date when the championship was Monterrey WCT, Mexico?
CREATE TABLE "singles_55_25_30" (
  "outcome" text,
  "date" real,
  "championship" text,
  "surface" text,
  "opponent" text,
  "score" text
);
SELECT MIN("date") FROM "singles_55_25_30" WHERE "championship"='monterrey wct, mexico';