This depends on the individual event but it is often 10am on a Friday. The on-sale timelines are determined by the promoters/producers and are shared with announcements on social media and by email. You can sign up to email alerts on our website.
Ticket agents often run pre-sales for artist and fan mailing lists, priority customers (O2 priority), and platinum customers (Ticketmaster) so please check separately with these sites for information, shortly after an event has been announced.