Definition
Tax-loss selling involves selling a security at a loss before December 31 so that the capital loss can be applied against capital gains realized in the same tax year, or carried back three years or forward indefinitely against other capital gains. For the loss to settle in the current tax year under T+1 settlement, the trade must be executed no later than the second-to-last trading day of the calendar year (the loss must settle by December 31). A key constraint is the superficial loss rule under ITA s.54: if the taxpayer or an affiliated person (spouse, controlled corporation) purchases an identical security within 30 days before or after the sale, the capital loss is denied and added to the ACB of the repurchased shares. Investors who want to maintain exposure to an asset class after a tax-loss sale commonly buy a different but correlated security (for example, selling one Canadian bank stock to realize a loss and immediately buying another) to stay invested while satisfying the 30-day window.
Source
Income Tax Act s.54 (superficial loss); ITA s.111 (loss carryover rules); CRA T4037