BirthDay.cs

public class BirthDay
{
    private const string Template =
        """
        Hey,
        
        {name}'s birthday is on {dob}, which is in {month}.
        
        Let's plan a party!
        
        """;
    private readonly Dictionary<string, string> _parameters = new();

    private BirthDay(string name, DateOnly dob)
    {
        _parameters.Add(@"{name}", name);
        _parameters.Add(@"{dob}", $"{dob:MM/dd/yyyy}");
        _parameters.Add(@"{month}", $"{dob:MMMM}");
    }

    public static BirthDay CreateInstance(string name, DateOnly dob) => new(name, dob);
    public override string ToString()
        => _parameters.Aggregate(Template, (sender, kv)
            => sender.Replace(kv.Key, kv.Value));
}

Example.cs

var result = BirthDay.CreateInstance("Mary", new DateOnly(1956, 9, 21)).ToString();