Balaram Purnima

The foremost manifestation of Kṛṣṇa is Saṅkarṣaṇa, who is known as Ananta. He is the origin of all incarnations within this material world. Previous to the appearance of Lord Kṛṣṇa, this original Saṅkarṣaṇa will appear as Baladeva, just to please the Supreme Lord Kṛṣṇa in His transcendental pastimes.
(SB 10.1.24)

Within the womb of Devakī is My partial plenary expansion known as Saṅkarṣaṇa or Śeṣa. Without difficulty, transfer Him into the womb of Rohiṇī.
(SB 10.2.8)

Balarāma appeared with Him as Lord Nityānanda.

(CC Adi 5.6)

Playing like a bull, Lord Balarāma fights with Kṛṣṇa head to head. And sometimes Lord Kṛṣṇa massages the feet of Lord Balarāma. He considers Himself a servant and knows Kṛṣṇa to be His master. Thus He regards Himself as a fragment of His plenary portion.
(CC Adi 5.136 – 5.137)