<aura:attribute name="objectsToInsert" type="Object__c[]"/>

cmp.set("v.objectsToInsert", objectsToInsert);
helper.insertObjects(cmp, cmp.get("v.objectsToInsert"));

var insertAction = cmp.get("c.insertObjects");
insertAction.setParam("objects", objects);
console.log("entries: " + JSON.stringify(objects));

"Day__c":"2016-12-07"

@AuraEnabled
public static void insertObjects(List<Object__c> objects) {
    System.debug(objects);
}

Day__c=null

@AuraEnabled
public static void insertObjects(String objects) {
    List<Object__c> objList = (List<Object__c>) Json.deserialize(objects, List<Object__c>.class);

    System.debug(objList);
}

@AuraEnabled
public static void MyMethodOfDoom(Date dayOfReckoning)
{
    if(System.today() == dayOfReckoning)
        delete [select Id from Contact];
}

let doom = new Date();

@AuraEnabled
public static void MyMethodOfDoom(String dayOfReckoning)
{
    if(System.today() == Date.valueOf(dayOfReckoning))
        delete [select Id from Contact];
}