You can search a column for a list of values. Here's a formula that uses that approach.
dt = New Table( "test", Add Rows( 12 ),
New Column( "Month", Character, "Nominal",
Set Values(
{"January", "February", "March", "April", "May", "June", "July",
"August", "September", "October", "November", "December"}
)
),
New Column( "Season",
Character,
"Nominal",
Formula(
spring_list = {"March", "April", "May"};
summer_list = {"June", "July", "August"};
fall_list = {"September", "October", "November"};
winter_list = {"December", "January", "February"};
If( Contains( spring_list, :Month ), "Spring",
Contains( summer_list, :Month ), "Summer",
Contains( fall_list, :Month ), "Fall",
Contains( winter_list, :Month ), "Winter",
);
)
)
);