For most undergraduate programs, your "Letter of Acceptance - Use for applying for Canadian Study Permit" should indicate two possible start dates for your studies. If you look in Box 26 of the Letter of Acceptance (Length of program), you will see two terms listed if this applies to your program.
When you apply for your study permit, ensure you use the start date that will give you the time necessary to have your application approved. If, for example, you receive your offer of admission in November, and it will take longer than a month to process your study permit application for the Spring term, then when you apply for your permit, use the Fall term as your start date. If you have questions about this, contact us at: firstname.lastname@example.org
If you are not able to begin your studies at the University for either term in which your offer was valid, you will need to reapply for admission.