This is the sort of question you only ask if you want an argument, and as no one else is biting.
While Robin is correct as regards millennium and centuries, in the mind of public opinion a decade is usually considered (whether right or wrong) as going xxx0 to xxx9. When you refer (say) to the 1920’s you tend to think of 1920 to 1929, perhaps because we’re lazy and only look at the “tens” figure, or because our brain starts to hurt when we try and workout how 1930 can possibly be referred to as being in the twenties.
It’s probably best to use whatever definition your target audience is happiest with, regardless of any legal or technical definitions, and if you’re having a “beginning of a new decade” party then you use the ambiguity to have 2 parties.