Scheduled reminders and thank you emails work off the date that the slot is on, based off of the Task Date Option. E.g. if you have a slot that is supposed to be on April 7th and you have a scheduled reminder set for one day in advance and a thank you one day after. At midnight on April 6th the system will look in your event for any slots scheduled to start on April 5th and will send the reminder (or whatever template you have chosen) to all the volunteers currently scheduled for April 7th. The same principle is applied to the Thank You emails. On midnight on April 8th, the system will look for any slots that ended on April 7th and will send the thank you email to any volunteers currently in those slots.