Go to Admin>Config>Core Set up and click on Holidays. You will see the dates that have been entered listed there.
If they are incorrect, use the delete button to remove them and re enter the new dates at the bottom of the page using the start and end date buttons. Don't forget to select the holiday type, then click the Add Holiday button to complete the action.
When adding the students holidays, you need to use the first and last date of that particular holiday.
If you are adding a training day, you'll need to either adjust the start or end date as appropriate.