Strictly speaking, every year starts a new decade. For example, 2007 started the decade of 2007-2016. A decade just means ten years. However, for many cultural purposes (such as to group events together) a series of decades is designated according to a regular pattern.
For most of the population, the significant decades are those beginning in a year that ends in zero, and running through to include the year that ends in nine. For example, 2010 to 2019 inclusive would be a decade.
Some people are sticklers for correctness. They remind us that there is no year zero in our calendar—the year called 1 CE (AD 1) directly follows the year called 1 BCE (1 BC). Therefore, the argument goes, the first decade ran from 1 CE to 10 CE, and all subsequent decades must follow the same pattern.
It’s all a bit theoretical, because our current calendar system didn’t exist back then. It was designed centuries later. Nevertheless, those who support the idea that decades must start with years that end in one can be quite insistent about it.
Those who don’t feel constrained by the lack of a year zero tend to prefer the convenience of defining each decade as those years that only differ in their last digit. This allows the convenience of referring to the decades by their “tens digit”: the nineties, the eighties, the seventies, the sixties, the fifties, the forties, the thirties, the twenties, and the tens. But what of the decade before the tens? It’s sometimes called the “noughties”, a somewhat jocular term.
Feel free to celebrate the start of the decade whenever you like. But take a tip from me: the “new decade” parties in in January 2010 will be more lively than the “pedants’ parties” in January 2011.
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