[Python] Find column name with highest value [duplicate]

On the Pandas dataframe below I use max to find the maximum value:

df["Max"] = df[['Day 1','Day 2','Day 4']].max(axis=1)

Car Day 1 Day 2 Day 4 Max
Car1 4 7 3 7
car2 8 2 1 8

What do I do to find when is the maximum value instead of the value itself as the table example below?

Car Day 1 Day 2 Day 4 When
Car1 4 7 3 Day 2
car2 8 2 1 Day 1

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