Annually
twice-a-day or it can be called semi-diurnal which relates to such occurrences as tides.
Usually those days are called holidays.
yes i'm positive it happens every year
Winter happens every year in December
An event that happens every other year is called a "biennial" event. This term is derived from the Latin "biennium," meaning "two years." Such events can include things like festivals, conferences, or exhibitions that occur once every two years.
weekly
Leap year. It happens in February. february, and it occurs on leap years
It happens every year
A leap year happens every 4 years, where February has 29 days instead of the normal 28.
Every year is an important year. It depends on what happens in it
Annual.
Happens every year. (Perennial)