Gets or sets the seasonal (gamma) parameter of Holt-Winters specifying how to smooth the seasonal component. Must be within the unit interval [0, 1]. A small value means that older values in x are weighted more heavily. Values near 1.0 mean that the latest value has more weight. NULL means that the HoltWinters function should find the optimal value of gamma.

public Nullable<double> Seasonal { get; set; }