There are due dates and other important dates you should know about. For example, did you know that even though income tax returns for your corporation are due 6 months after your year-end date, if you owe taxes, CRA may start charging interest after 3 months? Do you know how often you have to file a GST/HST return? This will depend on your income. Do you know how long you have to retain your books and records? What happens when the due date for filing a return falls on a Saturday, a Sunday, or on a public holiday? When are instalment payments due?

Don't miss those important deadlines!
http://www.cra-arc.gc.ca/tx/bsnss/tpcs/ll-dts/menu-eng.html