Not wearing a retainer for a month is likely to allow slight shifting of a persons teeth. Unfortunately, when someone gets their teeth straightened, we spent a lot of time discussing the treatment of moving teeth and complications but very little time gets spent on retention of the teeth.
It is important to understand what causes teeth to move in the first place. Our jaw bones, which anchor our teeth in place, are dynamic (they are always changing in response to forces). Our teeth have to balance the force of our lips (our teeth support our lips. Think of someone who takes their dentures out. Their lips sink in.). Our teeth also have to endure the force of our tongue as it moves around and when we swallow (every time we swallow our tongue lifts up and goes to the front of our mouth). Our teeth (more so our jaw bones) are able to resist these forces but only for short amounts of time. This is when we put in the retainer at night to help support the teeth and jaw bone.
Not wearing a retainer for a month will likely allow the teeth to shift but only slightly. The retainer is likely to go in but only by force (which should not be done). The good news is that the shifting of the teeth should be minor and not cause much of a difference aesthetically; so a new retainer can be made and just maintain the teeth in their current location. Hope this helps.
My best to you!
William F. Scott IV, DMD