The speaker is marking the birthday of someone who used to matter to them and no longer does. They insist the day "really has no meaning" to them, or "shouldn't," and that small correction gives the game away. The relationship was short, a few thousand days by their count, and it ended badly, leaving the speaker worse off than before.
What makes the poem sting is the list in the middle. The speaker says they can suppress memories, yet they can still recall how this person lifted them up and saw their flaws as "beautiful quirks." Being able to list those things is, in their own words, proof they haven't forgotten. The date has stuck "like glue," almost like a habit they never chose.
The mood is quiet, tired, and a little defeated. The plain, almost casual language makes the ending land harder: the day will pass, and it will just be "another day lonely."