Once you pick a year-end you pretty much have to live with it going forward. If you want to change the year-end you have to get permission from Revenue Canada. It is not automatic that they will grant the change in year-end. In the year of incorporation, you must choose the first year-end. It is any date within 12 months of the incorporation.
Most companies have year-ends that fall on the last date of a month.
Further, ending inventory factors into the calculation of net income. A lower ending inventory means lower taxable income and you therefore defer taxes. For retailers, it is the goods on their shelves. For manufacturers, it is the raw material, partial completed products and finished goods ready for shipment. If you have a December year-end you will be dealing with a busy accountant.