A disposition date may have different meanings depending on the context. In a financial context, the settlement date is the disposition date for stock and bond transactions on the stock market. The settlement date is usually three business days after the day your broker executed your sell order (trade date).
In the context of a criminal case, the disposition date is the date on which the outcome of a particular case occurred. Typically, sentencing is not included as a disposition. The disposition date is used for record-keeping purposes, but may also be a factor in penalties for subsequent offenses. Disposition dates may also be used in other contexts, such as filing of grievances.