SHIWASU is December in the Japanese lunar calender. We write it 師走 in KANJI characters, meaning "teacher is running". There are many theories to explain the origin of this word, and it is commonly said that the sound was there first, and the KANJI letters were applyed afterwards. Afterwards meaning 8th century (Heian period), is quite old anyway. In this theory, it is said that "SHI" (monk teacher) has to "HASERU" (run around) to chant the Buddhist sutra everywhere during this season, so the "SHI-HASERU" became shorter to "SHIWASU".
Many Japanese companies become very very busy in December, although the fiscal year usually ends in March. It will be a polite joke to say "Well, it is SHIWASU we might as well be busy" when you are having a business talk.