I remember the first time I flew Delta; I was so nervous about missing my window that I kept refreshing my phone. I eventually learned that the "sweet spot" for checking in is exactly 24 hours before your scheduled departure.
On my last trip to Seattle, I set an alarm for the 24-hour mark. I hopped onto the Delta Fly App right as the clock struck, and it was seamless. Doing this is the best way to snag a better seat or even snag a last-minute upgrade if you're lucky. If you need to modify your baggage details or add a Pet-in-Cabin request, that’s also the perfect time to do it. You can check in via the app, their website, or even the airport kiosk if you're old school, but the digital route saves so much stress.