Firewall rules can be scheduled so that they are only active at certain times of day or on certain specific days or days of the week.
Before you can apply a schedule to the rule, you must create a schedule under Firewall > Schedules. Then, when creating a rule, pick the defined schedule from the list.
Keep in mind that when the rule is inactive, the rules is not skipped -- the opposite action is applied. If you schedule a block, a pass is assumed at all other times. If you schedule a pass, a block is assumed.
In 2.0 this behavior has been changed, and the schedules rules will instead act as though they do not exist when the scheduled time is not active.