In Jainism, "Antariksha" or "Antarikshi" Parshvanath is not recognized as one of the 24 Tirthankaras. The 23rd Tirthankara is Lord Parshvanath (also spelled Parshwanath or Parsvanatha), who is a highly revered figure in Jain tradition.

Janam Kalyanak (birth anniversary) of Lord Parshvanath is celebrated on the 10th day of the dark half of the month of Paush (Paush Krishna Dashami) according to the Indian lunar calendar. This usually falls in December or January in the Gregorian calendar, but the exact date varies each year based on the lunar calendar.

If you meant a different figure, please clarify. According to Jain scriptures and tradition, there is no Tirthankara or prominent Jain figure known as "Antirskshi Paswanath."

Summary:

• Parshvanath Janam Kalyanak: Paush Krishna Dashami (10th day of the dark half of Paush month)
• Gregorian Equivalent: December or January (date varies each year)

If you require the specific Gregorian date for a particular year, please specify the year.