Yes, in Jain Dharma there are fasts and observances connected with Siddha bhakti and Siddha-related austerities.

Common examples:

• Upavās: complete fast
• Ekāsan: eating once in a day
• Ayambil: very simple, tasteless food
• Chauvihar / Tivihar: vow of fasting from certain foods or at night
• Navkārsi, Porsi, Sārdhaporsi, Purimaddha: time-based fasting disciplines
• Siddha Vandana / Siddha Stuti: devotion and prayer to Siddhas

If you mean a specific fast for Siddha in a festival or ritual context, Jain traditions usually do bhakti, jap, and upavās in remembrance of the Siddhas, especially on holy days. The exact observance can differ in Digambar and Shwetambar traditions.

If you want, I can also tell you the fasts related to Siddhachakra or Siddha Chakra Aradhana.