Shouldn't "We don't travel in the winter." also be accepted?

Already reported it, but "...THE winter," should be acceptable here. One of the most common mistakes Ukrainian and Russian speakers make in English is to omit articles, "a," "an," and "the," which don't exist in Ukrainian (or Russian).

Is '-ку' in 'взимку' a case marker or a suffix '-к-' + the case marker '-у'?