Definition:
Fixed Date Release is the known future date when team plans to deliver. For this scope of the release needs to be flexible.
In Agile, planning must be handled by controlling scope to meet the deadlines. Timebox, is a fixed start date and end date, in other words we are dealing with fixed end date. This is handled by varying the scope. As usually everything is not of high priority. High priority features are completed early in the release as scope is cut to release by the end date.
As we adjust the scope, making commitments as late as possible is also a technique where we can commit to deliver at the certain date which is agreed upon.
Further Reading:
http://www.innolution.com/resources/glossary/fixed-date-release
https://pm.stackexchange.com/questions/16372/how-do-you-schedule-delivery-dates-in-scrum
http://toolsforagile.com/blog/archives/1116/being-agile-with-a-fixed-release-date-commitment