For most individuals that report employment income, your deadline is April 30! This means that if you owe taxes, you must file your return by midnight on April 30, in order to avoid late filing fees.
However, if you are reporting income from an unincorporated small business, the deadline to file is June 15. But be aware, even though your return has the June 15 deadline, you still must pay the taxes owing by April 30 in order to avoid interest fees.
There are additional requirements to file as well: splitting pension income, re-paying OAS or EI benefits, you have a Home Buyer Plan or Lifelong Learning Plan or withdrawl from your RRSP.
There are many situations to be aware of, so it is best to consult a tax specialist if you are unsure.