5. There shall be a sitting of Parliament and of each legislature at least once every twelve months.
Prior to 1982, section 20 of the Constitution Act, 1867 set out a similar obligation for the Parliament of Canada. Section 20 was repealed by the Constitution Act, 1982, enacting the present Charter provision.
Section 86 of the Constitution Act, 1867 imposes a similar obligation on the legislatures of Ontario and Quebec. Presumably this obligation now must be read in conjunction with section 5 of the Charter. A similar obligation also applied to the Manitoba legislature under the Manitoba Act, 1870, but was repealed by the Constitution Act, 1982.
The guarantee of an annual sitting dates back to the mid-17th century in England. Prior to that date, long periods often passed without the monarch summoning Parliament. This was especially true under Charles I from 1629 to 1640. Various statutes, including the Triennial Acts of 1641 and 1664, were designed to guarantee that Parliament would be summoned on a regular basis. Following the second revolution, the Bill of Rights of 1689 also provided “that for redress of all grievances, and for the amending, strengthening and preserving of the laws, Parliaments ought to be held frequently”.
Article 25 of the International Covenant on Civil and Political Rights gives citizens the right “to take part in the conduct of public affairs, directly or through freely chosen representatives”.
Subsection 4(2) of the American Constitution provides that Congress must meet at least once a year.
There has been very little judicial commentary on this Charter provision. Presumably, the purpose of section 5 is to ensure that Members of Parliament and of legislatures carry out their democratic mandate, especially with respect to the principle of responsible government.
Commentary by the Supreme Court suggests that the obligation under section 5 applies to Parliament and the legislatures themselves: the legislature is called to sit by the Speaker and the action is purely internal to the legislative body (New Brunswick Broadcasting Co. v. Nova Scotia (Speaker of the House of Assembly),  1 S.C.R. 319).
Section 5 is not subject to override by the notwithstanding clause at section 33 of the Charter.