According to the folk calendar - Semin's day

This name of the holiday comes from the name of Saint Simeon, revered by the Orthodox on this day. It is customary to begin Semin Day by lighting a fire, which was made by hand. Semin's day falls in the middle of the first month of autumn; it is often associated with the beginning of "Indian summer". At this time the weather is still warm, but in some areas of the country it may become colder.