The guaranteed participation date is the last date on which an investor can buy a security and be certain of being able to take part in an elective corporate action. A trade executed on or before that date is due to settle in time for the buyer to hold the security, with the right to elect attached, before the event's deadline. It is set by counting back from the market deadline by the length of the settlement cycle.
The date matters when a trade fails to settle on time. If the seller has not delivered, the buyer can ask the seller to make the election on its behalf provided the trade was made by the guaranteed participation date and the request arrives before the buyer protection deadline. The dates are published with the event's election deadline in the corporate action notification for a voluntary corporate action.