Three months without formalities: the basic rule
Article R*421-5 of the planning code exempts from any formality constructions that remain in place for no more than three months. Service-Public applies this principle to temporary garden annexes (garden shelter, swing seat, light play structure), exempt from formalities and from the development tax and the preventive archaeology tax. A reception tent, also a temporary installation, falls under the same principle of article R*421-5.
A reception tent put up on Thursday and taken down the following Monday falls well within that frame. The period runs from setup to removal, so keep it in mind if the tent stays up for several weeks during a busy party season. When the period ends, the structure must be removed and the land returned to its original condition.