The properties and their acceptable values are outlined below:

Property Man. Card. URI Value
label rec 0..1 skos:concept xml:string
startDate rec 0..1 schema:startDate xsd:date (ISO 8601)
endDate rec 0..1 schema:endDate xsd:date (ISO 8601)

As stated, the Period class consists of only recommended properties. However, when providing a Period for your dataset atleast one of these recommended properties must be provided. An empty Period class is considered invalid.