SAML2 Metadata - Multiple Signing Certificates

I'm looking at a SAML IdP's metadata and it lists three unique certificates - 2 signing and 1 encryption.

    ...   
    <md:KeyDescriptor use="signing">
    <ds:KeyInfo xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
        <ds:X509Data>
            <ds:X509Certificate>
            </ds:X509Certificate>
        </ds:X509Data>
    </ds:KeyInfo>
    </md:KeyDescriptor>
    <md:KeyDescriptor use="signing">
    <ds:KeyInfo xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
        <ds:X509Data>
            <ds:X509Certificate>
            </ds:X509Certificate>
        </ds:X509Data>
    </ds:KeyInfo>
    </md:KeyDescriptor>
    <md:KeyDescriptor use="encryption">
    <ds:KeyInfo xmlns:ds="http://www.w3.org/2000/09/xmldsig#">
        <ds:X509Data>
            <ds:X509Certificate>
            </ds:X509Certificate>
        </ds:X509Data>
    </ds:KeyInfo>
    </md:KeyDescriptor>
    ...

I understand why it lists a signing and encryption certificate, but how do I determine which signing certificate to use in my service provider? Why are there two signing certificates at all?

Thanks in advance!