您需要在查询之前更改定界符:

You need to change delimiter before the query:

delimiter //

CREATE TRIGGER check_date BEFORE INSERT on Event
FOR EACH ROW
BEGIN
  IF new.date <= now() THEN
    SIGNAL SQLSTATE '45000' 
    SET MESSAGE_TEXT = 'Event cannot start in the past     event cannot start now. Choose an ulterior date';
  END IF;
END;//

delimiter ;

此外，在执行事件(触发)的整个代码块之前，请先结束 IF 语句

Also, end your IF statement before you END the entire block of code to execute for an event(trigger)