September represents one of the most demanding periods in the tax calendar for Portuguese companies, concentrating in a single month multiple obligations such as VAT, IRC, withholding taxes and Social Security contributions. This concentration creates significant pressure on company treasuries, which need to reserve a relevant portion of their liquidity in a short period of time. The impact is particularly relevant for small and medium-sized enterprises, whose financial management is frequently constrained by customer payment terms, salary costs and already committed obligations.
The main reason for this pressure is related to the VAT calendar. The VAT Code determines that periodic declarations relating to June, under the monthly regime, and to the second quarter, under the quarterly regime, are submitted in September. Added to this is the VAT declaration for July for companies under the monthly regime, meaning a company may need to submit two monthly declarations or one quarterly declaration in the same month. Payment of the assessed tax is due by September 25, and in 2026 the submission deadline moves to September 21, since the 20th falls on a Sunday.
When the VAT pressure begins to ease, the second IRC advance payment arrives, with a due date of September 30. Companies subject to IRC normally make three advance payments throughout the year, with September being the time for the second installment. The cumulative impact can be significant for companies that have to simultaneously bear VAT, IRC, Social Security, salaries and withholding taxes, all with very close deadlines.
The article warns that non-compliance can generate default interest and fines, increasing the cost of failures that could be avoided with proper planning. The main recommendation is to project all cash outflows in advance, identifying any liquidity needs and adjusting the payment calendar, inventory management, investments or customer collection. For an SME, a few days' notice can make a considerable difference to treasury.
