As with children who are only 1 when they've lived for a year... similarly at the very beginning [had we been sentient beings capable of such abstractions] we would've celebrated each and every milestone... ie, 1 day old, 1 week-old, 1 month etc.... and finally 1 year old when we would've then started the second year... at 1st January 2020 we start the new decade... at the end of the first year of that decade it will be 31st December 2020....
I thanks you....