Dear Raghu

A detailed Thread titled the Significance of Markali Thiruvathirai in line with the other two Threads of mine titled (1) Significance of Masi Maham and (2) Significance of Panguni Uththiram - presently appearing in this Web page will be posted two months later. A Thread titled the Significance of Thai Poosam too will appear very soon in this Web Site. Other titles in this series will appear couple of months later in this same website.

In the meantime I present below very briefly on the "Thiruvempaavai" (Markali Thiruvaathirai) as requested by you.
The month of Markali (from December 15th to January 15th) signifies the dawn of the Munpani Kaalam (early misty period), with the blowing of very cold breeze during the entire period.

It has been a practice in the ancient Tamil Nadu in the month of Markali - ten days before the day of the Natchatthiram the Thiruvaathirai, for the young (unmarried girls) to get up early in the morning before the sunrise and to purify - themselves with a bath in a river, and their minds free from impure thoughts, and go the temple of God Siva in groups, and offer their prayers of worship, requesting the God Siva to bless them with their future life partners - to be of kind hearted, good natured, understanding and providing love and care to them throughout their life time. This period of ten days worship ends with the tenth day falling on the Thiruvathirai Natchathiram day, mostly falling in conjunction with the full moon day which is said to be the very auspicious day to God Siva. The ten days of holy bath and intense worship to God Siva was known as the "Markali Neeradal" a practice which originated and was practiced in many regions in Tamil Nadu from the very early days.

Among the four foremost Tamil Saiva Saints of Tamil Nadu, the Saint Maanikkavaasagar during his time while on his way worshipping the holy temple Shrines of God Siva in Tamil Nadu came to the Thiruvannaamalai Temple Shrine during the month of Markali. There he saw many young girls getting up early morning and also waking up the other young girls in their neighbourhood, and going in groups to have the holy bath in the river, and to the worship of God Siva at the Thiruvannaamalai Temple.

For the Saint Manikkavaasagar who went shrine by shrine in Tamil Nadu worshipping God Siva singing holy verses of divine praise, this "Markali Neeradal" by young girls appeared as a new theme for his divine songs in praise of God Siva.

From the time these divine songs were sung by Saint Maanikkavaasagar which was titled as "Thiruvenpaavai Paadalkal", became the vogue thereafter to be sung in all Siva Temples and Religious Institutions during the ten days of worship in Markali prior to Thiruvaathirai Natchaththiram, and the name "Markali (river) Neeradal" gradually lost it's significance and it became to be known as the "Thiruvenpaavai Religious Festival" with Markali Neeradal in rivers, gradually turning into the Neeradal at their respective homes in the modern age.

The Vaishnavaite Thiruppaavai religious festival, also celerberated in the Month of Maarkali have the same significance, and the theme of the "Thiruppaavai Padalkal" sung by the Vaishnavite Tamil Saint Andal on God Vishnu too was based on the same theme of "Markali Neeradal" by young girls and the worship being done in the Vishnu Temples in Tamil Nadu.