Term used to describe a the period during the French Revolution. Despite the name, the "October Days" begin with the storming of the Bastille (July 14, 1789) - or alternately, with the adoption (on August 27, 1789) by the National Assembly of the Principles of 1789 (which, in the preface to the constitution of 1791, became the text we now know as the Declaration of the Rights of Man and of the Citizen).
The events of the "October Days" revolved around the person of Louis XVI, who was then residing at Versailles, dodging the issue of constituional legislation. Feeling the pressure, he called out the army. Provoked by this, the Parisian mob (in this instance, largely female) marched on Versailles on October 5, 1789. The following day, the king agreed to relocate to Paris.
Soon, the National Assemply followed suit, also relocating to Paris. In an effort to quell further unrest, the Assembly decided to declare martial law on October 21, 1789. This is unanimously regarded as the end date of the "October Days".