2026 Tax Calendar: What the Self-Employed and Small Businesses Can’t Afford to Miss

The 2026 tax year is unfolding for the self-employed and small businesses under two forces worth keeping firmly in mind: the usual calendar of quarterly filings and settlements isn’t letting up, and at the same time mandatory e-invoicing digitization, known as Verifactu, is rolling out. Whether you handle your own bookkeeping or rely on an advisory firm, you need to understand what’s changing and when, because tax deadlines are unforgiving and late-filing surcharges apply automatically, with no inspection required.

Verifactu and e-invoicing are already underway

The new verifiable invoicing system requires the software you use to issue invoices to meet specific technical requirements: full traceability, tamper-proof records, and in many cases, real-time reporting to the Tax Agency at the moment of invoicing. The approved rollout is staggered: first for companies subject to Corporate Income Tax, then somewhat later for the self-employed and other personal income tax filers. If your invoicing software, or your advisory firm’s, isn’t adapted yet, 2026 is the year to sort it out, since switching systems mid-year complicates quarter-end closing and can leave invoices in the wrong format.

On top of this comes the expansion of mandatory e-invoicing between businesses, which means reviewing not just how you issue invoices, but how you receive and archive every document.

The quarterly calendar shows no mercy

Beyond these regulatory changes, most of the fiscal year still revolves around the same milestones: the quarterly VAT settlement (Form 303), the personal income tax installment payment (Form 130 under direct estimation, 131 under modules), withholdings on employees and professionals (Form 111) and, where applicable, on rentals (Form 115) — all of them due within the first twenty days of January, April, July and October. On top of these deadlines come the annual summaries filed in January, such as Form 390 for VAT and Form 190 for withholdings, plus, where applicable, Form 347 for transactions with third parties in February.

Self-employed workers remain subject to the system of contributing based on actual net income, which means keeping income forecasts under review and, above all, staying alert to the adjustments Social Security makes once tax data from previous years is finalized: those who overpaid get a refund, those who underpaid get a bill, and both can arrive months after the year they relate to.

What this means for your business

In practice, this calls for some very concrete decisions. First, protect your schedule: set aside the days before each filing deadline to review outstanding invoices rather than leaving it to the last minute, since an error caught late means filing an amended return. Second, check as soon as possible whether your invoicing software is, or will be, Verifactu-ready before your turn comes up based on your business structure; switching software mid-quarter is avoidable with enough advance planning. Third, if cash flow is tight on the deadline itself, file anyway: it’s better to file without payment and request a deferral or installment plan than not to file at all, since the surcharge and penalty risk for failing to declare are considerably worse than the surcharge for deferring payment. And fourth, self-employed workers should watch for Social Security contribution adjustment notices as soon as they arrive, since the window to appeal or reclaim an overpayment runs from the date of notification, not from when the person happens to notice it.

At Zythos Business, we turn this calendar into something that doesn’t rely on anyone’s memory: tracking every deadline, reviewing the books before filing any return, and guiding the transition to Verifactu, so the self-employed and small businesses can focus on running their operations while we make sure the Tax Agency and Social Security never spring a surprise.