Restaurant Week refers to an organized promotion by a large number of a city or region's restaurants. During a particular time period, which may or may not actually be a week in length, the participating restaurants will put together special menus at a fixed price. Frequently the regular menu is available simultaneously. Ostensibly, these promotions are a method of allowing customers to sample a number of new restaurants without the enormous expense of trying them at normal times of the year, and without the risk of blowing hundreds of dollars on a tasting menu at a lackluster establishment. Most likely the promotional expenses and thinner-than-usual profit margins are considered an investment in repeat business. A side effect of these promotions is a temporary influx of customers who would be unlikely to dine at the participating venues for reasons of cost.
At least in Boston, Restaurant Week occurs twice a year and is two weeks in length each time. Lunch is offered at a price derived from the current year; e.g. in 2009, lunch is $20.09 plus tax and tip, and dinner is $33.09. Some participants only offer one of lunch or dinner.
For whatever reason, it seems that Yelp's Boston restaurants attract most of their bad reviews during Restaurant Week. It may be that the quality of the food is lower in order to take less of a financial hit. The quality of the service may go down as the servers may expect lower tips as a consequence of the presence of less desirable customers. New customers may simply have overly high expectations, punishing businesses for their inability to live up to a mythical ideal. Whatever the cause is certainly a matter for research.