NOW that April 21 has been decreed (we cannot be sure by whom) Secretary's Day in the territory, the Keeping Posted postbag is bulging with press releases from hotels and restaurants announcing special lunches on that day for this fine band. After all, it is they who ensure the wheels of the economy keep on grinding - especially while their bosses are out at long lunches! Correct us if we are wrong, but we thought the days of palming off the secretary with a lunch (together with the offer of a ''substantial'' pay rise usually made in the broom cupboard during the office Christmas party) belonged to the past. Today's secretaries, we are inclined to believe, won't settle for anything less than a cosy weekend at some quiet resort - and if the boss can make himself available to wait on them hand and foot, so much the better.