Day Seventy Seven

 is a part of the story in Firewatch.

Guide
Big day today. Head South from your lookout, crossing Ruby River, and then head west towards Cottonwood Creek. Access Cache Box 308 - this time, it's 5-6-7-8, not 1-2-3-4. Grab the new radio. Listen to the conversation.

Go North, past Medicine Wheel and through the canyon you named. Head up to Jonesy Lake, then take the trail North-East of the gate. Open the gate with your axe and continue to Wapiti Station. Examine everything in the tower and tent, and grab the wave receiver.

Following the beeping device all the way to the back of the tent, and clear the mess to find the black box and file on Henry and Delilah. Head back towards your lookout, and watch the cut scene. Follow the wave receiver, and take the path from your lookout to Medicine Wheel. On the map, right about the "E' on the word Medicine Wheel, you'll find a strange backpack and a key for Cave 452. Head back to the lookout for the placard for Day Seventy Eight.