After receiving an event invitation, you can propose a new time to the event organizer. You can propose a new time before or after replying to the invitation.
In the Calendar app in the toolbar, click New, then find your proposal in the notifications list. A calendar notification may also appear in the upper-right corner of the organizer’s screen, depending on the organizer’s Calendar settings in force click the event, click the My Status pop-up menu in the event info window, then choose a status. You can also add a comment to your reply that is seen only by the organizer.
If your declined events disappear from Calendar, and you want to see them again, choose View > Show Declined Events.