While in the "Event Details" tab of your event, you can change the dates that tickets are on sale under “Dates.” As long as your event is active and the current date is within the “Registration Opens” date and “Registration Ends” date, your tickets will be on sale. Once the Registration End Date passes your event will no longer be accessible to anyone other than you.
You can also set up different types of tickets to go on sale on different dates/times. To do this, go to the "Tickets" tab of your event. Select which ticket you would like to edit by clicking on the three dots on the far right of the row. Changing the "settings" of any ticket will allow you to change the dates that those particular tickets are on sale.