Due date and category-wise deadlines: GSTR-3B for March 2026 (last return of FY 2025-26) is due 20 April 2026 for monthly filers with aggregate turnover above ₹5 crore. QRMP filers in Category I states (Chhattisgarh, MP, Gujarat, Maharashtra, Karnataka, Goa, Kerala, Tamil Nadu, Telangana, AP, Andhra Pradesh, Union Territories of Daman & Diu, Dadra & Nagar Haveli, Puducherry, Andaman & Nicobar Islands, Lakshadweep): due 22 April 2026. Category II states: due 25 April 2026.
Year-end ITC reclaim — critical window: Under s.16(4) CGST Act (as amended), ITC for FY 2025-26 invoices can be availed up to the earlier of: (a) the due date of the GSTR-3B for September 2026 (i.e., 20 October 2026); or (b) the date of filing the annual return (GSTR-9) for FY 2025-26. Run a complete GSTR-2B vs books reconciliation for the full year — any eligible ITC not yet claimed should be availed in the March 2026 GSTR-3B or in April-September 2026 returns. After that window closes, missed ITC is permanently lost.
Rule 42/43 annual true-up: If a partial ITC reversal under Rule 42 (inputs/input services) or Rule 43 (capital goods) applies to your client, the final annual reversal computation for FY 2025-26 should ideally be completed in the March GSTR-3B. Any difference between provisional monthly reversals and the final annual figure must be adjusted before March GSTR-3B is filed. Reversal in any month after March also works but flagging in March avoids year-end reconciliation complexity.
Tax Liability Breakup confirmation: As per GSTN Advisory (17 March 2026), before filing GSTR-3B, the "Tax Liability Breakup, As Applicable" tab must be opened, verified, and saved (EVC/DSC). Especially relevant for March 2026 where year-end adjustments may generate auto-populated interest. Confirm the system-computed interest against your own calculation before confirming.
Checklist before filing March GSTR-3B: (a) Reconcile GSTR-2B with purchase register — claim any missed FY 2025-26 ITC. (b) Complete Rule 42/43 annual true-up. (c) Include any year-end credit notes and debit notes. (d) Confirm Tax Liability Breakup tab is opened, reviewed, and saved. (e) Discharge all outstanding GST liabilities, including interest on late payments. (f) Confirm correct bank account is linked for auto-debit (QRMP). March GSTR-3B is the single most important GST return of the year — treat it accordingly.