Māsa-kṣamaṇa has no single fixed start date for all Jains in 2026. The date is usually chosen according to your local Jain panchang, temple calendar, and guidance of your Acharya/pandit, and communities can differ by a day or two. ( jainknowledge.com)

If you want to align it with Paryushan 2026:

• Śvetāmbara Paryushan is commonly listed as starting on 8 September 2026 and ending on 15 September 2026. ( oshwalnairobi.org)
• Digambar Das Lakshan Parva is commonly shown as beginning around 15 September 2026. ( jainknowledge.com)

So, in short: start your Māsa-kṣamaṇa on the date approved by your local Jain sangh/acharya; if you want to connect it with Paryushan, early September 2026 is the relevant period. ( jainknowledge.com)