Theme release year
This can be a subject of some historical controversy sometimes, as some themes are old enough that it is not clear when they were first released (what is the first year of System, for example?).
Modern themes are often released at different times in different markets, or previewed one year but released early the next. As is the practice amongst many manufacturers such as the automotive and book publishing industry, "January" themes are nowadays actually available in stores in December of the previous year in key markets. Sometimes the difference turns on whether something was intentionally sold rather than accidentally, as there have been instances of inadvertent releases of merchandise.
Previews of new releases are eagerly awaited by fans and events such as the Nuremberg, London and New York Toy Fairs, San Diego Comic-Con and even fan conventions feature displays of new sets and minifigures.