A holiday period is used to define the start and end date of a company holiday year. The period parameters were set when you setup your SMB account and roll annually from the original start date.
Holiday periods are organised into Holiday Period Schedules, which allow you to run multiple concurrent holiday periods - periods that start and end with different dates. For example, one group of employees may have their holiday calendar running from January to December, while another group may have their own calendar running from April to March.
In order for employees to record absences in the future, you will need to ensure that you have a holiday period defined to cover the dates they wish to take holiday. If a holiday period has not been defined, then the employee requesting an absence that falls outside of the defined periods will receive a message informing them that a period has not yet been defined.