Do I pay the capital gain tax at the time of sale (i.e. at closing) or after declaring the capital gain on my tax declaration for the year in which I sold the property?

Any capital gain realized in a taxation year is reported at the time of filing the tax return for the relevant taxation year and any taxes owing for the year (that would include a tax payable component attributable to the taxable gain reported) is paid at the time of filing the return (by the appropriate tax filing deadline date for the return).