Thaipusam (தைப்பூசம்)
Thaipusam is an annual festival occurs in the month of Thai aka Pushya/Pausa. It is dedicated to the worship of Lord Kartikeya, Shiva and Devaguru (Brihaspati).
The word “Thaipusam” in Tamil (South Indian language) represents the full moon in Pushya nakshatra. According to the puranas Goddess Parvati bestowed Kartikeya the legendary weapon (Vel) on this day to win the battle against asuras in which he became victorious and saved the devas. Also it's one of the most auspicious days to worship Shiva and Brihaspati because he’s the presiding deity (adhi devata) of Pushya nakshatra and Shiva is also considered as Guru in many ways. Kartikeya, Shiva and Brihaspati all represent the Guru tattva.
That's why Pushya is chosen to celebrate them all at once, another nakshatra that represents all of them is Shravana which is directly opposite to Pushya. Both these nakshatras are having the quality of 9th house (Guru, luck, higher knowledge, father etc.).
Thaipusam is celebrated mainly by people of South India (states of Tamilnadu, Kerala, Karnataka, Andhra Pradesh). It's also a famous festival in many parts of the world especially in Southeast Asia such as Malaysia, Singapore, Srilanka, Mauritius etc.
Worship
You can worship Lord Kartikeya the whole day from Sunrise to Sunset, you can also perform puja by fasting and end it with chanting the mantra “Om Sharavana Bhavaya Namah” for 108 times to Kartikeya or by doing milk abhishekam (pouring milk) to Shivling and chanting “Om Namah Shivaya”.
Remedies for Mars and 9th house
Kartikeya is the adhi devata of Mars. Mars is the physical energy, action, passion, protection, bravery, skills, efforts, blood, bone marrow etc. So, people with weak Mars can also harness the positive energy of this day to energise their Mars. Since it has strong significations of Guru its also one of the best days to energise your 9th house.