Upvote:1
You do not need a visa for a short stay, because you are a US citizen. Anyway, the earliest you could apply for a visa starting in December would be September, because you cannot apply more than three months in advance (Schengen Visa Code, Article 9).
US citizens are subject to the "90/180" rule, meaning that short stays are limited to 90 days within any 180-day period. Days authorized under your long-stay visa do not count in that calculation, so you should be fine.
This is laid out in Article 6(2) of the Schengen Borders Code:
Periods of stay authorised under a residence permit or a long-stay visa shall not be taken into account in the calculation of the duration of stay on the territory of the Member States.