Implemented in Common Lisp.
Tested in Steel Bank Common Lisp (SBCL), not guaranteed to work in any other LISP :)
For a given day N, load the dayN/dayN.lisp file in your LISP environment and run either (run-dayNa "./path/to/input.txt") or (run-dayNb "./path/to/input.txt") to run the part A and part B solutions for each day on a given input file.
For example, to run day 1, using SBCL
# cd day01/
# sbcl
... sbcl initialization ...
* (load "./day1.lisp")
T
* (run-day1a "./input.txt")
<the answer>
Obviously it's no fun to look at the answers if you plan to do it yourself! So solve the problem first and then take a look. Or if you're not interested in doing it, just take a look for fun :).