A business day convention is a rule written into a financial contract that says what happens when a scheduled date falls on a day that is not a business day. Banks and payment systems are closed at weekends and on public holidays, so a payment due on such a day cannot be made; the convention specifies the day to which it moves. It applies to bonds, loans, deposits, swaps and other derivatives.
The convention works together with two other terms. The first is the definition of a business day itself, which names the financial centres or payment system whose calendars are used. The second is the day count convention, which determines how interest is calculated for the period. All three must be known to compute a payment, and they are stored as part of the reference data for every instrument.
The main conventions
The standard conventions are set out in the definitions published by the International Swaps and Derivatives Association and are used across markets:
- Following. The date moves to the next business day.
- Modified following. The date moves to the next business day unless that day falls in the next calendar month, in which case it moves back to the preceding business day.
- Preceding. The date moves to the previous business day.
- Modified preceding. The date moves to the previous business day unless that falls in the previous calendar month, in which case it moves forward.
- No adjustment. The date is left as stated, although the actual payment is still made on a business day under the contract's payment terms.
Modified following is the most widely used for swaps and floating-rate instruments. Its purpose is to keep a payment within the month in which it was scheduled, which matters for month-end and quarter-end accounting.
An example shows the difference. A payment scheduled for Saturday 31 May 2025 would move, under the following convention, to Monday 2 June. Under modified following, because 2 June is in a different month, it moves instead to Friday 30 May.
Adjusted and unadjusted periods
A second question is whether moving the payment date also changes the amount of interest. If the calculation period is adjusted, interest accrues up to the new date, so a payment delayed by two days carries two more days of interest and the next period is shorter. If the period is unadjusted, interest is calculated to the original scheduled date and the same amount is simply paid later or earlier.
Fixed-rate bonds usually use unadjusted periods: each coupon payment is the same regardless of when the date falls, and the holder receives no extra interest for a delay. A floating-rate note (FRN) and the floating leg of an interest rate swap usually use adjusted periods, so the amount reflects the exact number of days. The combination is stated in the terms, for example "modified following, adjusted".
Which calendars apply
A day counts as a business day only if the relevant places are open. The contract names them, for instance London and New York, or refers to the days on which the euro area's central bank payment system operates. Where several centres are listed, a day must be a business day in all of them. A related term in payments is the banking day.
Holiday calendars change. Governments declare one-off holidays, sometimes at short notice, and events such as state funerals or severe weather can close a financial centre. Contracts generally apply the calendar as it stands when the date arrives, so a payment date can shift after the trade was agreed.
Where the conventions are used
Business day conventions apply to more than payment dates. They adjust the effective date and termination date of a swap, the dates on which floating rates are fixed, option exercise and expiry dates, and the maturity date of loans and deposits. The full list of dates for a bond is laid out in its coupon schedule, generated by rolling forward from the first date or backward from maturity and then applying the convention to each one. Rate fixing dates are covered under reference rate reset.
Some instruments add an end-of-month rule: if the first date is the last business day of a month, every later date is also the last business day of its month.
In over-the-counter derivatives, the conventions are agreed in the confirmation, which incorporates the standard definitions under the parties' master agreement. For bonds they are stated in the terms and conditions.
Operational importance
Small differences in convention produce real differences in cash. A swap hedging a bond will leave a mismatch if the two use different conventions or calendars, because their payment dates and day counts will occasionally diverge. Treasury systems, custodians and paying agents must hold the right convention for every instrument, or interest will be calculated for the wrong number of days and payments expected on the wrong payment date.
Typical errors include using a single-centre calendar where two apply and confusing adjusted with unadjusted accrual, which surface as cash breaks or disputes over bond accrued interest. For this reason the convention is treated as a mandatory field in instrument reference data, and holiday calendars are sourced from specialist data providers and updated continually.