PSA: This xkcd was released 6 days ago and nobody posted it. It is in fact allowed for other people to post new xkcd comics to this community too, you don't have to wait for me to do that.
Relevant to this idea, I use these apps to let me know when things like the International Space Station, meteor showers, aurorae, etc are visible overhead:
I have an app called Star Walk 2 that does something very similar. Notifications about cool shit happening and then helps you to look in the right direction for it, including ISS fly-bys.