Sun’s annual transit over Scorpio is called the month of Kartika. Sun (Surya) transits over Scorpio annually from the mid of November to mid of December.
Sun aspects Krittika nakshatra when it enters Scorpio and the New Moon of this month usually falls on krittika so the influence of krittika is high in this month, thus the name. Krittika (Pleiades) represents the "first spark", the tiny spark that lights up everything. It has the combined energy of Mars and Sun.
So, the theme of this month will be
the creative spark that lights up the inside and outside of you
finding the unknown
bringing the unknown to light
Note: The theme may also work in opposite ways since Scorpio is unexpected (it all based on your personal chart).
It is said that this is the darkest month of every year. Because the next month (Margashira) represents early morning so the time before that should be the darkest. So we need fire to light up our consciousness. That's why Karthigai deepam (light of krittika) is celebrated on the full moon of this month. According to Puranas Lord Kartikeya was born through the light of krittika to destroy darkness and to re-establish dharma in the world.
Scorpio is a fixed water sign. It represents still (immovable) waters that are deep such as lakes, wells, ponds, swamps and muddy waters. Sun’s movement over these regions evaporates more water and brings back more rain. Scorpio has three nakshatras namely Vishakha, Anuradha and Jyeshta. Vishakha represents rain and thunder, Anuradha represents stored waters and Jyeshta represents muddy waters.
Note: Because Sun and moon are deciders of the earth’s weather and Scorpio is the natural 8th house of sudden changes and permanent transformations, it can either bring more water or the opposite.