Skanda: evolution of the Great Deity
It is well known that Skanda is always associated with Kṛttikā Nakṣatra (Pleiades cluster - 6 stars), as His name itself is Kārtikeya. He is also associated with the Viśākhā Nakṣatra (Libra cluster - 4 stars). Today's celebration Vaikāsi Visākam (viśākhā star in vaiśākha month) as the birth star of Skanda has a very ancient Vedic-Puranic connection. But this festival is observed only in Tamil Nadu widely as per my knowledge and not in any other parts of India.
Viśākha is not just a name of Skanda, because of His birth star. In later Vedic texts and in the epics, Skanda and Vishakha are actually considered as brothers or even twin deities.
Many Grihya Sutra and Dharma Sutra texts mention worship of Skanda in his various forms in rituals. For example, as per Āgniveshya Grhyasūtra ablutions were to be offered to, among other deities, Skanda, Vishākha and Sasthī, the consort of Skanda. Baudhāyana Dharmasūtra (2.5.7) speaks about tarpana (religious offerings of water) to Skanda in his different forms, Skanda, Sanmukha, Vishākha, Mahāsena and Subramanya and also his pārsada-s and pārsadī-s (attendants). Skanda-yāga to the twins Skanda-Vishakha is mentioned in some Atharva texts. Specific Sāma Vedic hymns are also said to be chanted for them.
Patanjali's Mahabhashya (2nd century BCE) to Panini grammar Ashtadhyayi: The commentary for sūtra 5.3.99 says that images of Śiva, Skanda and Vishākha were no longer being sold commercially, which was the case during the times of the Mauryas, and that during his time these images were used only for worship. This is cited as one of the most ancient proofs for image worship in the Hindu tradition.
Mahabharata - Vana Parva: When Kārttikeya displayed his enormous capabilities the devas instigated Indra who after initial hesitation declared war against Kārttikeya. Indra was completely defeated. The thunderbolt hurled by him at Skanda pierced him on the right side. A new god Viśakhā came out from the right side of Skanda.
Mahabharata - Shalya Parva: Rudra, Uma, Agni and Ganga request Brahmā to perform the rites of making child Kumāra the general of the army of celestials which is done on the banks of river Sarasvatī. Here Skanda assumes four forms - Sakha, Visakha, Naigameya and Skanda. Sakha goes to Ganga, Naigameya to Agni, Vishakha to Parvati and Skanda to Rudra. All the four claiming to be the parents of Kumāra achieve satisfaction.
In the Later Puranic legends, all these deities merge into one Great Deity of Skanda, the Devasenapati.
Greetings to all devotees celebrating the festival today.
Material in this post is mostly taken from the research articles archived in the website
http://murugan.org/ Image: Skanda statue from Kannauj, 8th century CE.