It was happiest day of my life vs It was the happiest day of my life

Some examples from the web:

• You know, when you first came out of your mom, well... it was the happiest day of my life.
• It was the happiest day of my life until the day you came to replace him.
• It was one of the happiest days of my life.
• Gib, you just helped make this the happiest day of my life.
• Some of the happiest days of my life have been spent...
• This is the happiest day of my life.
• Happiest day of my life when I saw that Raptorjump into the rendezvous point.
• Or even better, fall face first into it, and then it really will be the happiest day of my life.
• I was reliving the happiest days of my life. I didn't want it to end.
• I always knew my wedding would be the happiest day of my life.
• Point is, when I marry Myk in March, it'll be the happiest day of my life.
• But the day we closed escrow, you said it was the happiest day of your life, and that made me feel really good about it.
• Louise Weiss said in her address to the European Parliament that it was the happiest day of her life.
• The happiest day of my life was the day we got married.
• This is the happiest day of my life.
• The time I have spent at Parks and Rec have been the happiest days of my life here in the government.
• You know, one that you'd wear forever, so it can remind you of the happiest day of my life.
• The first few months after we were married were the happiest days of my life.
• Just think, the happiest day of her life.
• He has spent the happiest days of his life.

Some examples from the web:

• It was the happiest day of my life until the day you came to replace him.
• You know, when you first came out of your mom, well... it was the happiest day of my life.
• It was the happiest day of my life.
• It was one of the happiest days of my life.
• But the day we closed escrow, you said it was the happiest day of your life, and that made me feel really good about it.
• Louise Weiss said in her address to the European Parliament that it was the happiest day of her life.
• Tom said yesterday was the happiest day of his life.
• And as the day wore on, David thought it was the happiest day of his life.
• The day that I proposed, You told me that that was the happiest day of your life.
• I mean, the day he became a teacher was the happiest day of his life.
• At the time, I thought that was the happiest day of my life... but every day just got better and better.
• And then I would ask them a number of questions like what was the happiest day in their life, what makes them happy.
• Gib, you just helped make this the happiest day of my life.
• Some of the happiest days of my life have been spent...
• This is the happiest day of my life.
• Bambi, when you broke up with Danni, you said it was the happiest day of your life!
• Or even better, fall face first into it, and then it really will be the happiest day of my life.
• I was reliving the happiest days of my life. I didn't want it to end.
• I always knew my wedding would be the happiest day of my life.
• Point is, when I marry Myk in March, it'll be the happiest day of my life.