In Jain practice, on vrat days like aatham pakhi (the eighth day of certain holy observances), the general guideline is to follow a simple, sattvik diet and to avoid foods that are considered stimulants or non-vegetarian. Two important dietary rules to keep in mind:

• Avoid root vegetables: potatoes, onions, garlic, carrots, beets, radishes, etc.
• Avoid foods that break the spirit of ascetic/fasting observance: excessive sweetness, fried items, and anything that complicates the vow of restraint.

A sandwich is usually not suitable for aatham pakhi if it contains root vegetables or onion/garlic, or if it includes ingredients that are not considered sattvik. If you can make a sandwich with strictly sattvik, onion- and garlic-free fillings (for example, cucumber, tomato, paneer or cheese that is acceptable in your tradition, without root vegetables or onions/garlic) and a plain bread that has no eggs and is considered acceptable in your Jain community, some people may permit it. However, many strict observances would still discourage sandwiches on such days because of common fillings like onion/garlic or root vegetables.

There are differences between Digambar and Shwetambar practices. Both generally discourage root vegetables and non-sattvik ingredients during vrata, but the exact allowances can vary by community and local guru or achar. If you want a precise ruling for your situation, please check with your local Jain temple or guru for the specific aatham pakhi rules you follow.

Note: If your question refers to a specific regional custom, share that detail and I can align the guidance to that tradition.