Throughout the year; all year round.
この地域は年中暖かい気候だ。
This region is warm throughout the year.
年中無休で営業しています。
We are open year-round without holidays.
果物は年中手に入るわけではない。
Fruits are not available all year round.
Always; all the time; continually (used adverbially).
彼は年中忙しそうにしている。
He seems busy all the time.
年中体調が悪いと言っている友人がいる。
I have a friend who says they're unwell all the time.
年中行列ができる人気店だ。
It's a popular shop that has lines all year round.
During an era or a particular year of an era (used in historical/literary contexts).
年中に起きた事件について記録が残っている。
There are records about incidents that occurred during that year/era.
A kindergarten class level (nenchū) — the middle year in preschool (second-year kindergarten).
うちの子は来年、年中に上がる予定だ。
My child will move up to the second year of kindergarten next year.
年中組は年少組よりも活動が活発になる。
The nenchū class becomes more active than the nen-shō class.