Your policy will start on the 1st of the next month (i.e., if you purchase your policy on 15 November, your coverage will start on 1 December if you are eligible for the cover).
However, for policies purchased within the last 4 working days of any month, your policy will only start on the 1st of the subsequent month, due to the processing time required to administer your policy. (i.e., if you purchase your policy on 30 November, your coverage will start only on 1 January).