Answer

There is no single fixed date for starting Siddhitap in Chaturmas for all Jain traditions. It is usually started on an auspicious day within Chaturmas, after proper vidhi and with guidance from a guru, Acharya, or knowledgeable Jain elder.

Explanation

• Chaturmas is a very sacred period for austerity, devotion, and discipline.
• Siddhitap is a serious tapasya, so it should be begun with a clean mind, firm sankalp, and proper ritual method.
• In some traditions, people begin it soon after Chaturmas starts; in others, the start day is chosen according to local custom or a specific panchang.

Spiritual Understanding

The important thing is not just the calendar date, but:

• right intention
• proper vow
• discipline in observance
• guidance from the right source

In Jain Dharma, tapasya becomes especially meaningful when it is done with humility, श्रद्धा, and नियम.

Takeaway

Start Siddhitap only on a suitable and traditionally approved day during Chaturmas, ideally after consulting your Jain sangh or guru. If your local tradition has a fixed practice, follow that exactly.