In Jainism, “Atham” usually means the 8th tithi (Ashtami) of the lunar fortnight. Many Jain families keep special functions, pooja, or vows on Atham – but your sentence is incomplete.

Do you mean:

• Why there is a function in your family/derasar on Atham?
• Any specific Atham like Chaitra Atham, Kartik Atham, etc.?
• Some particular vrat (fast) or pooja done on Atham?

If you tell me which Atham and which type of function (pooja, bhaavna, swamivatsalya, etc.), I can explain its Jain significance in simple terms.