This is actually not true. It's a common misunderstanding due to the wording on the CIC website. matthewc, creator of the e-CAS automatic tracker, explains this here: http://www.canadavisa.com/canada-immigration-discussion-board/is-ecas-updated-only-on-tuesdays-t47537.0.html;msg455282#msg455282.
His post on another forum about the frequency of e-CAS updates by day and hour: http://www.roadtocanada.com/forums/showthread.php?tid=2640&pid=50402#pid50402.
Short version (quoted from matthewc's post): "Conclusion: It actually looks like early Monday morning (between 2am - 6am EST) is the most common time for updates to show up, but updates do happen every day." So if you don't get an update on a Tuesday or Thursday, don't worry; it doesn't mean you can't get an update on some other day that week. It really can be any day of the week.