During the winter season, climate doesn't normally allow any outdoor sports in most countries, at least in Europe. That's when the indoor season starts for the decathletes as well. Of course, indoors you can't practice all the events, e.g. it would cause some problems to throw a discus or javelin. That's why the number of disciplines is reduced from 10 to 7.
Heptathlon events are in the following order: 60 m - Long jump - Shot put - High jump - 60 m hurdles - Pole vault - 1000m
Every year there is either a World or a European Championships, mostly carried out in early March.