BJP national president Nitin Nabin, speaking in Patna, sent greetings for Chitragupta Puja, observed by the Kayastha community and people whose work involves writing.
He called Chhath Puja one of the most sacred festivals in Bihar's cultural history, saying its spirit is visible across India and abroad.
Ahead of the festival, Chief Minister Samrat Choudhary said 90 of the state's 103 ghats are completely safe for devotees, while 13 have been marked as unsafe or unsuitable.
Two independent MLCs from Bihar, Bansidhar Brijwasi and Rahul Raj, have switched to the BJP, and JDU, its NDA partner in the state, has expressed unease. State JDU chief Umesh Kushwaha pointed to BJP leaders who were expelled and then taken back within 15 days.
Both seats were contested by JDU as part of the alliance. The BJP maintains that the pair only offered support and never formally joined.
A planned joint press meet by Abhishek Jha and Anand Pushkar on Monday was called off after talks with JDU leaders and CM Samrat Choudhary.