local-time is a development library for manipulating date and time information in a semi-standard manner.
You can see the code:
https://gist.github.com/juan-reynoso/f5f881fc39b69d171741cd3b459f82e4
In order to run the function get-timestamp-data you need to load local-time.
(ql:quickload "local-time")
And using local-time package
(in-package :local-time)
LOCAL-TIME> (get-long-weekday-from-timestamp)
"Tuesday"
LOCAL-TIME> (get-long-weekday-from-timestamp :timestamp (timestamp+ (now) 1 :day))
"Wednesday"
LOCAL-TIME> (get-long-weekday-from-timestamp :timestamp (timestamp+ (now) 2 :day))
"Thursday"
LOCAL-TIME>