The 2026 e-Invoice calendar, and what to do in each of the next three months
A month-by-month plan for businesses in the final phase — what to fix now, what can wait, and where the rejections usually come from.
If your business crosses the final phase threshold this year, the work splits neatly into three months. Most of the pain we see at go-live comes from one of two places: customer master data that was never cleaned, and invoice fields that nobody mapped because they live outside the accounting system.
Month one: find out what you’re actually missing
Pull ten invoices you issued last month — the messiest ten, not the tidy ones. Check each against the mandatory fields. You are looking for the fields your team fills in by hand, because those are the ones that will fail validation at volume.
Nine times out of ten the gap is the buyer TIN and the classification code — not the integration.
Month two: clean the master data
This is the unglamorous month, and the one that decides whether go-live is quiet. Work through:
- Buyer TINs and registration numbers for every active customer
- One classification code per product or service line
- Credit-note and refund flows, which are usually undocumented
- Who is allowed to cancel a submitted invoice, and within what window
Month three: submit in parallel
Run live submissions alongside your existing process for two full cycles. Keep a log of every rejection code — the pattern tells you which mapping is wrong, and it is almost always a single field repeating.
| Month | Priority |
|---|---|
| October | Gap check on ten real invoices |
| November | Master data and code mapping |
| December | Parallel submission, rejection log, staff SOP |
If you’d rather not run the gap check yourself, send us one sample invoice and we’ll come back with the list.