From my observations, below is a calendar of traditional American culture. Emphasis on traditional because America is made up many different cultures, someone of which do not participate in anything listed below.
Vacation planning. Holiday season is over, time to throw something on the calendar to look forward to. First 'bank' holiday is MLK. A nice 3-day weekend to break the stress of the post-holiday craziness at work. Also, NFL playoffs.
Valentine's Day, the Super Bowl and Mardi Gras.
St Patrick's Day, Northern States starts to thaw from the winter. Baseball spring training. March Madness. In High School there is Prom and that is about as American as anything. Yea, other countries have it as well but going to prom is very American.
Easter, Passover. NBA playoffs begin. Baseball season begins.
May - August (Summer season)
Memorial Day (unofficial start of Summer) and 4th of July weekends - BBQ, beach, cruises, pool, taking that vacation you booked in January. Family reunions. Some schools start wrapping up and with that comes Graduation season.
Labor Day (unofficial end of Summer); back to school for kids on a traditional schedule
October - November (Harvest season)
Halloween and start of football season, World Series. Homecoming. In some parts, hunting season begins. It all culminates with Thanksgiving.
Shopping season begins which culminates with Christmas and New Years.
I wasn't born into American culture but was immersed in it at an early age. In elementary school, my classmates would celebrate things like Halloween and Thanksgiving which I knew nothing about at the time but was fascinated by it all. It's because of this that I paid closer attention to the details than others who were born into it.
If I were to describe American culture in one word, it would be: celebrate. Americans are forever looking for a reason to celebrate something. And I don't say that with any negative connotations.
If I were to say what phrase best describes American culture, it would be: What's next? That's the driving force behind everything. When one thing is over, the planning and anticipation of the next thing begins. We are always looking forward to what's next.