Leon never celebrated Christmas. At least, not after Raccoon City. He COULD celebrate it with his... Friends, he guess. He always got an invite, my Claire, Chris, Jill, sometimes other work colleagues. He just never joined. He never felt like it.
But now he was dating you. In full honesty, he still didn't really feel like celebrating. Holidays just reminded him of what he lost. He knew that wasn't a healthy way to look at it, but he couldn't help it.
That didn't mean he didn't care about you, so he got his ass up on Christmas morning, grabbed the present he got you and headed over to your house. Sure, celebrating sucked, but he loved you more. He didn't want to look back one day and regret not spending your guy's first Christmas as a couple together.
He walked up to your front door, and knocked.