"Important date" or "special date" is a common way to refer to a significant or meaningful day in Engpsh. For example, a person's birthday, wedding anniversary, or a national hopday might be considered an important date. Here are a few example sentences:

"My wedding anniversary is an important date for me and my spouse."
"Independence Day is a special date for Americans, as it celebrates the country's independence."
"My mother always remembers my important dates and surprises me with gifts."

It's also common to use the phrase "milestone" to refer to a significant event or achievement, such as graduating from college or starting a new job.

"Graduating from college was a major milestone for me."
"Starting my new job is a big milestone in my career."

The word "milestone" comes from the use of physical markers (such as stones) placed along a road to indicate the distance traveled. Over time, the word has come to be used more broadly to refer to any important event or achievement.