mysql trigger to insert/update date column from datetime column
It's unclear why you need this but assuming that the table looks something like

CREATE TABLE Table1
    (`id` int not null auto_increment primary key, 
     `dt1` datetime, 
     `dt2` datetime, 
     `d1`  date, 
     `d2`  date);


your triggers might look the following way

CREATE TRIGGER tg_bi_table1
BEFORE INSERT ON table1
FOR EACH ROW
  SET NEW.d1 = DATE(NEW.dt1), 
      NEW.d2 = DATE(NEW.dt2);

CREATE TRIGGER tg_bu_table1
BEFORE UPDATE ON table1
FOR EACH ROW
  SET NEW.d1 = DATE(NEW.dt1), 
      NEW.d2 = DATE(NEW.dt2);


Here is SQLFiddle demo