I started working on a contract basis from Aug 2024 - essentially becoming self-employed.Can I defer tax on Aug-Dec income by selecting a fiscal year end date in 2025?

If you started your business in in 2024, your first fiscal period ends in 2025, and you will report a business income for the first time in 2025, fill in the T1139 form to request an election for your 2024 return. You would then report your business income in 2025. But in 2025, you'll not only be reporting income from your first year of business, you'll also be reporting the income from your year end up to Dec 31. So, you would be paying tax on 18 months of income, which may put you in a higher tax bracket. You may want to speak to an accountant or tax expert before choosing to do this.