Override Error Messages

You can override the default message for a validation on a per property basis. Here is how you configure the extend call:

ko.validation.rules['mustEqual'] = {
    validator: function(val, otherVal) {
        return val === otherVal;
    },
    message: 'The field must equal {0}'
};
ko.validation.registerExtenders();

// If not modifying the default message, you would call like this:
// viewModel.PercentSum.extend({ mustEqual: 100 });

// If you want to override the message, you need to move 
// your validation parameter into the "params" field
viewModel.PercentSum.extend({
    mustEqual: {
        params: 100,
        message: "Percentages must add to 100"
    }
});

Starting with version 2.0.0 the validation message can contain multiple placeholders {0}. For instance, the following custom rule takes advantage of this feature.

ko.validation.rules.between = {
    validator: function(value, params) {
        var min = params[0];
        var max = params[1];
        
        value = parseInt(value, 10);
        
        if (!isNaN(value)) {
            return value >= min && value <= max;
        }
        return false;   
    },
    message: 'Value must be between {0} and {1}'
};

ko.validation.registerExtenders();

// Use the rule
viewModel.someValue.extend({between: [10, 100]});

See it in action