The following are additions to be inserted in lst8-2.c to add the extra
functionality.

After the line:

    void print_bottom_asterisks (int sun_date, int last_day);

in function print_calendar_month, insert the line:

    void headings(void);

The after the line:

    sun_date = 1 - start_weekday;

in the same function, insert:

    headings();         /* Display day abbreviations above columns. */

Then add the following to the end of the file:

/********************** headings: ************************
  On entry: SQUARE_WIDTH is greater than or equal to 3.
  On exit:  The days of the week abbreviations (Sun, Mon etc.) have been
            displayed so that they will center above the calendar columns.
*/

void spot(char day[]);

void headings(void) {   /*%%%%%%%*/
    putchar('\n');
    spot("Sun");
    spot("Mon");
    spot("Tue");
    spot("Wed");
    spot("Thu");
    spot("Fri");
    spot("Sat");
    putchar('\n');
}

/** Spot: Displays a day abbreviation centred in SQUARE_WIDTH columns.
          day must be a three-character string.
*/

void spot(char day[]) {
    int lead_size;
    lead_size = (SQUARE_WIDTH - 3) / 2;   /* No. of spaces before the name */
    putout(lead_size, ' ');
    printf("%s", day);
    putout(SQUARE_WIDTH - 3 - lead_size, ' ');
}