It is a common expression to be used at this time of the year.
The current year has mainly passed by, your plans starts to fall into the calendar of the year to come, but we are not that far yet – a sign that you are trapped in between years, or as the Germans say ‘zwischen den Jahren’ (‘between the years’).
Today the expression is mainly used to define this time of year between Christmas and New Years’ Eve, as some companies do not work, or work less and employees usually get some days off. However, history do has some more to add to this apparently self-explaining expression.
It happens that early Christians have had new Years’ Eve and Christmas all together on the 6th of January. By that time, in an attempt to facilitate the conversion of pagans to Christianity, the pope then transferred Christmas to the 25th of December. The point was that this was a pagan holiday marking the the winter solstice according to the Julian calendar. The correlation of both holidays was then seen as a way of spreading the faith in a more effective way.
Trouble started when in the 9th century the church made another change: New Years’ Eve was transferred from the 6th of January to the 25th of December. One can only imagine a transition period in the medieval times when with a poor communication system between regions people might have been celebrating New Years’ Eve with 12 days of difference. A time in which, people literally experienced a kind of in between years, depending merely on their location and access to the noble court.