You will have to expect some mass loss but in a month, very little.
What you will be able to do though is regain it quicker, as muscle has a type of membrane around it called a fascia, which expands when you build muscle, but practically doesn't shrink, if at all, when muscle atrophies, so when you try to regain muscle it has no resistance initially slowing the growth rate down, until you recover to where you were before and then any additional muscle building will incur resistance from the fascia.
Your body needs a break here and there, but the most important thing is that you get back into your routine and don't let 1 month turn into a whole year.
Maintaining a healthy diet during your time off will be essential.
That way when you go back in January 1 you are picking up closer to where you left off.
To be honest is not that bad to take a break of 1 month since the muscle will have full time to recover.Also you already did the job so the muscle keeps the memory for you so by the time you come back you will be fine!! ...And even better once you come back your body may respond faster ...you will lose some mass but nothing to be worried about it...you will get it back for sure!!
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I wouldn't take a month off from working out if I were you, especially considering that you cannot store fitness. If you need a break, just take a few days off from exercising.