Dwarves will occasionally go on break. Breaks last about 14 days and dwarves tend to take breaks every few months. Dwarves on break will behave as though they are jobless, and will wander between their bedrooms and public meeting places. There is no way to force a dwarf to work while on break, therefore the best use of this time is boosting the happiness of said dwarf. A high quality dining room, statuary, zoo, or meeting area will help in this respect. If a dwarf can be confined there by burrows, a danger room can still be used while the dwarf is on break, making this another productive use of the time spent on break.
Breaks probably do you more bad than good. Idling dwarves are never a good thing, as they do no work, build social skills, and tend to build friendships that can lead to temper tantrums later on if someone really well-liked passes away. On the other hand, they do give dwarves time to examine furniture and raise their happiness.
Disciplined dwarves are theorized to take either shorter or less frequent breaks.