Enum Naming Convention - Plural

Microsoft recommends using singular for Enums unless the Enum represents bit fields (use the FlagsAttribute as well). See Enumeration Type Naming Conventions (a subset of Microsoft's Naming Guidelines).

To respond to your clarification, I see nothing wrong with either of the following:

public enum OrderStatus { Pending, Fulfilled, Error };

public class SomeClass { 
    public OrderStatus OrderStatus { get; set; }
}

or

public enum OrderStatus { Pending, Fulfilled, Error };

public class SomeClass {
    public OrderStatus Status { get; set; }
}

I started out naming enums in the plural but have since changed to singular. Just seems to make more sense in the context of where they're used.

enum Status { Unknown = 0, Incomplete, Ready }

Status myStatus = Status.Ready;

Compare to:

Statuses myStatus = Statuses.Ready;

I find the singular form to sound more natural in context. We are in agreement that when declaring the enum, which happens in one place, we're thinking "this is a group of whatevers", but when using it, presumably in many places, that we're thinking "this is one whatever".

The situation never really applies to plural.

An enum shows an attribute of something or another. I'll give an example:

enum Humour
{
  Irony,
  Sarcasm,
  Slapstick,
  Nothing
}

You can have one type, but try think of it in the multiple, rather than plural:

Humour.Irony | Humour.Sarcasm

Rather than

Humours { Irony, Sarcasm }

You have a sense of humour, you don't have a sense of humours.