Excuse my ignorance, but what is a 5-day pad?
Its a pad that is used for 5 days then destroyed and replaced with another different pad with different codes.
The pad contains the five letter codes in play for that period. It goes like this (pretend this is Russian): AABBF BNNFT AACRG GFBBA SDFAA. These letters are read aloud over the radio (ANNA ANNA BORIS BORIS FEOTER etc) usually on a secure encrypted channel of a multichannel system. Decoded it would look like: General wants American pizza tonight. Occasionally there are dummy words inserted and other misdirections - like that pizza could mean an arty barrage at first light.
Contrast with US forces at the time: “Hey Brigade, this is tank unit 5, platoon 4 from the old 79th. The five of us are off hwy 274 near that old farm house with the funny windows and the old shell hole. We are in the field just behind the place under the only apple tree - you can’t miss us. Say hello to LT. Bob for us.” (actual transmission - close as I can recall something 50 years old, recorded, transcribed, and stored somewhere in NSA’s vaults).