The year is not defined as a multiple of days.
The year is the time for the Earth to orbit the sun once. This is not a constant amount of time. So it is not defined in seconds or days or any other length of time. However it only varies a little.
The (synodic) day is the amount of time for the Earth to rotate once relative to the Sun. This is also not a fixed amount of time. But like the year only varies a little.
Time is measured in seconds. one second is 9,192,631,770 cycles of the radiation produced by the transition between two levels of the caesium 133 atom. This is among the most stable clocks we know. The second can be redefined if a more stable clock exists.
Now for the practical aspect of calendar forming it is convenient to have a simple rule that anyone can apply and can decide, without the need to make careful observations, which day is in which year. To this end the Gregorian Calendar gives a fairly simple but accurate approximation to the true astronomical year.
The disadvantages of using astronomical observations to decide the first day of the year are significant. Politically, who makes the observations? Russia? China? Japan? Botswana? In theory it would mean that I could not know which day a year was in, until after that day had been reached, and the observations done. In practice there would be no difference from the Gregorian calandar for many centuries; so why bother.
To summarise: The astronomical year is approximated by the Gregorian year. This is simple to use. It is accurate for most calendar applications. It allows for the calendar to be extended into the future. The Gregorian calendar has wide acceptance among many countries. If precise timekeeping is needed then one doesn't use "years" instead one uses the SI unit of time, the second.