Q4 2026 Tax Calendar: What You Can’t Afford to Miss

Every year-end brings the same message for freelancers and small businesses: tax deadlines don’t wait, and last-minute rushes tend to be expensive. In 2026, with the Tax Agency stepping up the automatic cross-checking of data between VAT, withholdings and instalment payments, keeping on top of the tax calendar is no longer just a matter of good housekeeping. It has become a direct way to avoid penalties and formal notices. It isn’t only about remembering dates, but about understanding which obligation falls due when and which information you need to have ready in advance.

Most deadlines for freelancers and SMEs revolve around the quarterly returns: Form 303 for VAT, Form 130 or 131 for IRPF (personal income tax) instalment payments for those taxed under direct or objective estimation, and Form 111 if you have withholdings on employees or professionals. If you have employees, there is also the management of Social Security contributions, which in recent years has incorporated adjustments tied to the self-employed person’s actual income, meaning you need to review your contribution base more often than before. In addition, as the year moves towards its close, the annual information returns take centre stage, such as Form 190 for withholdings or Form 347 for transactions with third parties. They are filed early in the following year, but the preparation (reconciling figures, reviewing suppliers and customers) is best started before the current year ends.

On top of this comes a regulatory context that keeps evolving: mandatory business-to-business e-invoicing continues to move through its implementation timetable, and the Immediate Supply of Information system (SII) and verifiable invoicing systems keep expanding the level of detail the tax authorities require in real time. For an SME, this means that mismatches between what has been declared and what has been invoiced are detected sooner and leave less room for explanation. You don’t need specific penalty figures or dates that haven’t yet been officially confirmed to see the trend: the tax authorities are enforcing with ever more automation, so up-to-date bookkeeping is no longer optional but the only sensible way to operate.

What this means for your business

In practice, this translates into several concrete decisions. First, review the calendar for the upcoming quarterly and annual returns now, and block out in your diary not only the deadline itself but also a buffer of several days beforehand to close the books and catch any discrepancies while there is still time to react. Second, if your business generates many transactions with third parties or you work with new suppliers, start cleaning up that information now: reaching January with a full year of data still to be reconciled is the most common source of errors in Forms 347 and 190. Third, if you are self-employed and your revenue has changed significantly compared with the contribution base assigned to you, this is a good time to consider an adjustment before the year ends, avoiding unpleasant regularisations. And fourth, if your business is affected by the new e-invoicing or verifiable-system requirements, check now whether your invoicing software meets the technical requirements, rather than finding out once they are mandatory and there is no room left to adapt.

At Zythos Business, we support freelancers and SMEs with precisely this kind of forward planning: we keep your books up to date throughout the year, not just in the weeks before each filing, so that every quarterly return and every information obligation arrives with the numbers already reconciled and no last-minute surprises. If you would like to see how your particular situation fits into the tax calendar ahead, we’re here to help you plan it with time to spare.