Today is the Big Girl’s last day of second grade. She’s thrilled, but for me, as always with these milestones, it’s bitter-sweet. The days are long, but the years are short.

One of my resolutions is to “Be a storehouse of happy memories,” and one tradition that I started years ago, almost accidentally, was taking a photograph on the first and last day of school each year. I have the Big Girl (and starting next year, the Little Girl) hold up a sign that says “Last day of second grade—June 14, 2007” or whatever.

It makes a great keepsake. I wish that I’d been organized enough to start a special photo album, just with these pictures, so we could more easily see the changes over the years. I didn’t think to do that, so they’re just sprinkled in the albums among the other pictures, but they’re still fun to see.

Research shows that a good way to boost happiness is to reflect on happy times in the past, and looking at photographs—or any other memento—is a good way to prompt your recollections.

Also, although taking a photo is easy and takes only a second, the tradition of making the sign and taking the photograph adds a little bit to the special quality of these first/last days, and that’s important to me, because I want my children to understand the importance of school.

I make extra copies and send them off to the grandparents, too. So little work, so many happiness resolutions fulfilled!

From 2006 through 2014, as she wrote The Happiness Project and Happier at Home, Gretchen chronicled her thoughts, observations, and discoveries on The Happiness Project Blog.