Before revision: In parliamentary democracies, while heads of state (or their representative) in such systems of government may formally issue and accept invitations, they do so on the advice of their heads of government, who usually decide on when the invitation is to be issued or accepted in advance.
After revision: In parliamentary democracies, while heads of state (or their representative) may formally issue and accept invitations, they do so on the advice of their heads of government, who usually decide on when the invitation is to be issued or accepted in advance.