Last season, United recorded the Football League's top attendance in 2007-08 when 38,256 saw their final-day win over Gillingham on May 3.
With the club's seasonal average of 23,341 higher than all but four Championship clubs and also four Premier League sides - Blackburn, Bolton, Portsmouth and Wigan - Simon Grayson said: "It's brilliant the way the fans keep turning out and if we get another positive result on Saturday, we'll go into the play-offs on a really good run and full of confidence. I'm sure the fans will also turn out for the home and away legs in massive numbers, whoever we play and we want to reward them by getting to Wembley."