The convention helps the counterparties to understand when payments will be made for each trade.
There are two possible cases: Also, the spot date cannot fall on a US holiday for any USD currency pair.
However, foreign exchange trades can settle on this day (e.g. GBP/JPY on 4 July) but are considered FX outrights.
Time to expiry is usually quoted either as "overnight" or in terms of a number of days, weeks, months or years.
In general, the expiry date can be any weekday, even if it is a holiday in one, or both of the currencies, except 1 January.