Anniversary
noun, suffix
When you wrap around the entire circumference of the year you've come back around to the start of it. This is the anniversary.
This word is commonly used with numbers, like 10周年 (10th anniversary), or combined with other words, like 周年事業 (anniversary project).
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
周年記念イベントの準備で忙しい。
We're busy preparing for the anniversary event.
トーフグは、来年で十周年になるので、コウイチは周年プロジェクトを予定しています。
The next year is Tofugu's 10th anniversary, so Koichi is planning an anniversary project.
明日は私たちが付き合ってから、十五周年目の記念日です。
Tomorrow marks the 15th anniversary since we started dating.