It's your brain playing tricks on you, not a message from beyond. What you're describing is called apophenia - when you're looking for something, your brain becomes obsessed with patterns and sees them constantly. Before, you were seeing this date without noticing it, now it's jumping out at you everywhere because you're actively looking for it.
It's not that you're imagining things, it's just how the brain works. If you start paying attention to dates containing 7s for example, you'll see them everywhere too. That's called confirmation bias - once you believe something, you search for evidence that confirms your theory and ignore everything else.
As for your grandmother, keep her memories close, it's totally valid to think about her. But these repeated dates are just probability and your brain not leaving anything to chance. If it's stressing you out, stop deliberately looking for the date for a few weeks - you'll see it'll fade from your radar.