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.
We're busy preparing for the anniversary event.
The next year is Tofugu's 10th anniversary, so Koichi is planning an anniversary project.
Next Sunday we’re celebrating Tofugu’s 15th anniversary at McDonald’s.