The most intense forms of exercise - heavy weight sessions, sprinting (HIIT sessions of any form), or God forbid, distance running - benefit from 48-72 hours of "recovery." This is age-dependent because youthful bodies can handle almost anything.
People over 35 should not do weights on the same muscles two days in a row. If you do heavy weights (eg powerlifts) twice weekly, even then it's best to focus on different lifts each session. "Overtraining" can be a problem for obsessive exercisers.
Recovery means getting protein you need, good sleep (reparative growth hormone operates during sleep), and maybe making sure to roll out your muscles.
"Recovery" does not mean taking a day off from physical activity. How it is done depends on age and level of fitness. For example, fit 35-75 year-olds can use an hour of calisthenics with hand weights as a day of "active recovery" from weight-lifting, but unfit people might just benefit from a long-slow hour of non-cardio "cardio" as recovery. People have to listen to what their body is telling them, but not to their "lazy voice." It can be difficult to tell the difference.
We usually think of 45 minutes of calisthenics as active recovery for fit people under age 75 or so. Calisthenics sessions do not require more than 24 hours of recovery. Indeed, in my gym there is a cohort of around 40 men and women of all ages who do nothing more than 6-7 calisthenics 6 am classes each week. It clearly works well for their fitness, but lacks the bone and muscle strength components of strength training.
Recovery is one reason for the design of the Maggie's fitness program. Weights, Calis, Cardio, rinse and repeat and take one day for sports or hiking to enjoy your improving condition.
What about days off entirely from activity other than walking around? Such days hardly need to be planned, because life interferes regularly enough with our virtuous routines.