Craft task send mails at specific date
Might I suggest that a Craft task might not be the best way to pull off what you're trying to achieve.

By default, they will be triggered by the next control panel request, no matter when that occurs.

Sure you could build in some timing logic in your plugin, or set the runTasksAutomatically config setting to false and have a cron job hit the task at your set schedule, but that all seems overly complicated.

Just setup an independent cron job to fire when you need your logic to run and have it hit a custom plugin's controller's action and run your logic directly outside of Craft's task implementation.