EmbeddedMap<TKey, TValue> — a collection keyed by a property of the value

public class OpeningHours {
    public Guid Id { get; set; }
    public string DayCode { get; set; } = string.Empty;   // "mon", "tue", …
    public TimeSpan Opens { get; set; }
    public TimeSpan Closes { get; set; }
}

public interface IVenue {
    // ...
    [EmbeddedMapProperty(KeyProperty = nameof(OpeningHours.DayCode))]
    EmbeddedMap<string, OpeningHours> Hours { get; }
}

KeyProperty names the property that holds the key. It must be declared on the closest common base of the allowed value types, and its type must be TKey; the model builder checks both. To key by the value's id instead, set KeyType to KeyPropertyType.NodeGuidId or NodeIntegerId. Without an attribute, a Guid or int key means the value's id, and any other key type is an error.