Reason is mainly vrat (tapas) and smaran of Bhagwan Parshvanath’s janma‑kalyanak, not “kheer” itself.

In the traditional Posh Dashmi vrat (Magshar/Posh vad 9‑10‑11), many Śvetāmbar Jains (especially Gujarati) follow this pattern:

• Day 1: Ekāsan of sweet water / sharbat
• Day 2: Ekāsan of kheer
• Day 3: Ekāsan of simple full meal

This three‑day vrat is said to be based on the tapas of Śrāvaka Surdatt in honour of Parshvanath Bhagwan’s janma‑kalyanak.

Why specifically kheer ekāsan on the middle day?

1. Madhyam (middle) and saumy (gentle) food

- Kheer is simple, soft, easy to digest, and sattvik. - It is between very light (only liquid, day 1) and a full solid meal (day 3). - So it keeps the body supported but still maintains a feeling of vrat and control.

1. Focus on tapa, not taste

- Even though kheer is “sweet”, the rule is only once (ekāsan), limited quantity, no variety. - This creates niyam, saiyam, and tyāg – main aim of Jain vrata.

1. Smaran of Parshvanath’s kalyanak and Surdatt’s vrata

- By following the same type of three‑day pattern, the devotee does bhakti + anusmaran (remembrance) of those old tapas. - The belief is that such niyam helps shuddhi of bhav, reduction of passions, and punya useful for the path to moksha.

1. No major Digambar–Śvetāmbara doctrinal difference

- The tattva‑arth (inner meaning) is same: - control of senses - remembering Parshvanath Bhagwan - doing tapa with shraddha. - Exact food‑rules and local customs (kheer vs. only upvās/āyambil etc.) can differ by region and sampradāy.

So, kheer ekāsan on that ātham/second day is just one step in a 3‑day progressive vrat, meant to balance health and tapas, and to keep the mind in bhakti and saiyam for Posh Dashmi.

You can read more about the general significance of Posh Dashmi here: