how can i use switch with a string input with c?

I'm new programing in C, i want to do a program that convert the months to the number of the month using switch.

For example:

Input:

"january"

output:

"1"

Here what i do:

void function(char number[]);

int main()
{
    char xm[20];

    printf("Month:");
    scanf("%s", &xm);

    function(xm);

    return 0;
}

void function (char number[20])
{
    switch (number[20])
    {
        case 'january': printf("1");
                        break;
        case 'february': printf("2");
                        break;
        case 'march': printf("3");
                        break;
        default:
            printf("error");
    }
}