"Authorized personal drop" (APD) came into being as an offshoot (concession) of "authorized leave." We used to get 5 and then 3 days of authorized leave. I believe that it was negotiated in the contract in lieu of holiday pay. With authorized leave you could drop a 3 day trip and get paid for it with your 3 days. If you dropped a 4 day then you were out one days pay. I think authorized leave made it's exit in contract '96 but I could be mistaken. Authorized personal drop took its place.
With an authorized personal drop, you can drop any length of trip as long as the reserves meet the min threshold required in the contract to do so. On certain major holidays you are contractually excluded from being able to APD. They are listed in the contract. With that being said, if you want a major holiday off, try the APD. I have gotten Xmas off in the past using the APD option.
Bottom line as far as dropping and picking up trips using the APD. Only use the APD to drop the day you really want off. Then, if you want to pick up a trip before or after that day you can.
Denny