It happens the same way every time. On May 1st, you think, "My mom's birthday is May 14th. I have plenty of time to get a card." On May 10th, you think about it again. On May 15th, you wake up in a cold sweat, realizing you completely missed the day.
This cycle is one of the most socially devastating aspects of ADHD. In neurotypical culture, remembering a date is equated with love and respect. When you forget a birthday or an anniversary, other people read it as "you don't care enough to remember." But the brutal truth is that you do care. In fact, you probably thought about the birthday six times in the preceding weeks. What failed was the exact moment of execution on the actual date.
The ADHD brain suffers from severe 'time blindness.' It does not feel the steady progression of days. Time is binary: it is either "Now" or "Not Now." A birthday two weeks away is "Not Now," so it lacks urgency. By the time the actual date arrives, your working memory has been overwritten by immediate, high-dopamine tasks (work emergencies, answering a text, finding your keys), and the birthdate is completely pushed out of your consciousness.
You cannot fix this by "trying harder to care." You cannot fix it with a paper planner that you forget to check. You must accept that your brain is wholly incapable of tracking abstract time, and entirely outsource this responsibility to automated, interruptive external systems. The goal isn't to fix your memory; it's to make your memory irrelevant.