Just because you didn’t workout for a few weeks don’t go back to the gym
and go hard right off the bat like you haven’t missed a beat. You’re just
asking for injuries.

These injuries can be immediate or delayed. If delayed, it means you think
you got away with it, but few days later you don’t know why your knee hurts, or
why your back hurts. 
The few “hard core” workouts you execute right off the bat will
not make a difference to the results you’re after other than increase your
chance of injuries. Come back into the game steady. Do half of what you would
normally do before the holiday break, then slowly build your way up from there
every other day. 
What will make a difference to getting and keeping great results is your
regularity and consistency in working out, not your “one-hit-wonders”
here and there.
Let’s put it this way, your workout regime is like being in a relationship.
You can’t just start and stop; you can’t expect to spend the next few weeks
showing your partner your respect and affection and then stop just because you
got lazy. The relationship won’t last the test of time. Even if it does, it
won’t be a truly fulfilling one. 
This is the same with your workout regime. It can’t be a temporary thing.
If you choose it as part of your lifestyle, it only works best when you make it
a permanent thing.

It doesn’t have to be hard core, it doesn’t have to be everyday or all the
time. Just be regular. Be consistent. Even if it’s twice a week, this is more
effective in producing long-lasting results than if you were to spend only
a short period of time (few months) working out hard 4-6 times a week.