Perhaps you didn't train during a holiday, perhaps you trained less during the last lockdown or perhaps were not able to train because... you know. Life happens sometimes. A big concern for those who enjoy sports, especially if you have specific goals in training, is that we may grow weaker or *gasp* less fit during these periods. Is that the case?
ADAPTATION
Your body always tries to adapt to what you put it through. Put through heavy strength training and your muscles and bones get stronger. Put it through cardio and your heart and lungs get better at providing your muscles with oxygen. Put it through skill practice and you will improve that skill. (Well... we hope.)
The opposite is also true. A simple example is what we see with injured people. If people break an arm and wear a cast for six weeks, they will lose a lot of strength and muscle in that arm. The reason is simple - the body is not being put through any type of stimulus, regardless if that's heavy strength training, a long CrossFit workout or simply doing daily chores. It also happens in a more subtle way. If you're a CrossFitter but you have not done any squats in a while, your squat strength may get worse while you're still improving other aspects of your overall fitness.
HOW FAST DOES THIS HAPPEN?
Of course it takes time to 'detrain', otherwise nobody could get fitter from 2 workouts a week. After all, you'd be training 2 hours a week and detraining for another 166 hours a week. Luckily, that's not how it works!
General strength and basic endurance take a long time to build, but they also take a long time to go away. Not counting peak performance like 1RM deadlifts or 'best marathon times', you can maintain most of your strength and endurance for 3-4 weeks before it starts breaking down significantly. This is, of course, assuming that you just don't exercise but still do your daily stuff. If you lie in bed all day, like being seriously ill or injured, it would go a bit faster.
Muscle mass, muscular endurance and 'short term endurance' like we see in CrossFit deteriorate faster. Usually after around 2 weeks.
Peak strength, like with a 1RM deadlift usually deteriorates after 1 to 2 weeks. Explosiveness, power and speed deteriorate after 3 to 5 days already.
WHAT DO I DO WITH THIS INFORMATION?
The good news is that regaining fitness goes faster than building it up from nowhere. The other good news is that small amounts of exercise, even in suboptimal conditions, will slow down deterioration. This means that even if you do half of what you normally do at half the intensity, that is still better than doing nothing, regardless whether we're talking about speed, strength or health benefits! Literally the only exception would be if this somehow hurts your recovery from an injury or something.
Knowing all the above, you know that you can maintain a lot by:
Doing speed work like low-rep box jumps or sprints just 1 or 2 times a week
Do a CrossFit-like workout once a week
Do strength training once every 2 weeks (per muscle group or movement pattern) Do basic, aerobic endurance work once every 2 weeks
Is that going to make you a lot stronger and fitter? Probably not, but it does show that you only need a relatively small investment of time to prevent yourself from getting weaker during times where you are busy, less motivated or have less access to training facilities.
Long story short: MOVE. Everyday. Move frequently and do what you can. Anything that does not hurt you is good. Check the UnScared home workouts, go for a brisk walk in the park, practice your push-ups at home since you can improve those without any material at all, do a yoga session with 'Yoga With Adrian' on YouTube, walk your dog more. MOVE. Your future you will thank you.