The spot date is the value date of an FX spot transaction: the day on which the two currencies are actually exchanged between the parties. By market convention it falls two business days after the trade date, written T+2. A few currency pairs settle in one business day, the best known being the US dollar against the Canadian dollar.
The date is worked out by counting forward and skipping weekends and bank holidays in the financial centers of both currencies. The gap exists to give both sides time to confirm the trade and send payment instructions across time zones. The spot date is the reference point for the rest of the market. Forward contracts are priced as an adjustment to the spot rate for a later date, and positions that are not to be settled are moved to the next day with short swaps such as tom-next.