The Fitbit Flex can act as an alarm. When acting as an alarm, the Flex will vibrate at the time you set it to do so. The Flex supports up to 8 concurrent alarms, although the more alarms you run the more the alarm function drains the battery of the Flex. To set an alarm, log into the Fitbit portal and go to http://www.fitbit.com/settings/alarms. Once there, click on the Add Alarm button.
From the Set an alarm screen, enter a time and then whether the event should repeat (which brings up a menu of checkboxes for each day of the week). If repeating, check the appropriate box for each day.
Click Save once the event is ready and then sync the Fitbit (e.g. through the app on your phone).
When the alarm time is reached (btw, alarm times are based on the time zone you defined in your web client) the Fitbit will vibrate. Alarms automatically recur after 9 minutes. Double-tap on the Fitbit when you see the LED lights flash to disable the alarm. One light in the middle LED means that the alarm was disabled.