When serializing Rule class, https://github.com/droolsjbpm/drools/blob/5.5.x/drools-core/src/main/java/org/drools/rule/Rule.java#L122, the write and read external methods does not write and read calendars properties.
This causes that, if there is a rule which has a calendar in guvnor, for example,
rule "CalendarTest"
calendars "my cal"
no-loop true
dialect "mvel"
when
then
System.out.println("CALENDAR IS FIRIUNG!!!");
end

when rule is retrieved, calendars are not took into account.