Upvote:2
Changing your mind or slightly altering your plans is not illegal per se and the visa sticker won't mention your original plan. On the other hand, your visa will have a period of validity and that's the only thing that matters. If, as is likely, you are issued a visa valid from December 15th then you are out of luck and cannot present yourself to the border five days earlier (if entering by plane, you would be denied boading). If for some reason your visa starts earlier or if you change your plans to travel later then it's perfectly fine.
Upvote:4
I asked the German Consulate in Toronto and here is their answer (relevant excerpt from the full screenshot here):
With regards to your question, a tourist visa will be issued as per submitted documents at your visa appointment. In other words, if you submit documents for a trip to Germany from December 23rd to 31st, the visa will be issued for exactly that time period. It is not possible to enter the country at an earlier date in this case.