While British legislation is now cited in the format Northern Ireland Act 1998 (UK), s 5, pre-1963 legislation (which is more likely to be referred to in Canadian courts than the newer legislation) is slightly more complicated. 1
Older British legislation is cited using a regnal year, e.g. Statute of Frauds, 1677, 29 Car II, c 3. The regnal year consists of the shortened version of the monarch’s name (in this case Car II for Charles II where “Car” is the shortened version of Carolus, the Latin for Charles) preceded by a number or numbers indicating the year of reign of the legislation. While the McGill Guide says to use Roman numerals for the monarch, it does not explicitly say to use the Latin name so you may also see the regnal year cited in the format 29 Cha 2. Note that “Jac” is used as the shortened version of James (Jacobus).
Wikipedia has a handy table of regnal years of British monarchs. The regnal years (mostly) start of the first day of the monarch’s reign but not always. The Wikipedia table has an interesting list of the exceptions.
You will also see the regnal year used in older Canadian legislation (e.g. federal statutes enacted before 1867 and provincial statutes enacted before the province entered Confederation).
1. The old citation system was changed by the Acts of Parliament Numbering and Citation Act 1962 since it was seen to be “inconvenient”. Mansfield Park is quoted in Hansard in explanation of the change.
Note: this was edited on March 1, 2023 to show “Car” as the preferred citation rather than “Cha”.