To begin or start.
“Your new position will commence on the first Monday of next month.”
“The renovation work will commence once the permit is approved.”
'Commence' is a formal alternative to 'start' or 'begin' commonly found in employment and official letters in Reading Part 1, such as when stating a job start date or when a policy takes effect. Note the exact date mentioned, since this is often tested.
Synonyms: begin, start, initiate