Calendar

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int width = 80, year = 1969;
int cols, lead, gap;

const char *wdays[] = {"Su", "Mo", "Tu", "We", "Th", "Fr", "Sa"};
struct months {
    const char *name;
    int days, start_wday, at;
} months[12] = {
        {"January",   31, 0, 0},
        {"Februray",  28, 0, 0},
        {"March",     31, 0, 0},
        {"April",     30, 0, 0},
        {"May",       31, 0, 0},
        {"June",      30, 0, 0},
        {"July",      31, 0, 0},
        {"August",    31, 0, 0},
        {"September", 30, 0, 0},
        {"October",   31, 0, 0},
        {"November",  30, 0, 0},
        {"December",  31, 0, 0}
};

void space(int n)
{ while (n-- > 0) putchar(' '); }

void init_months()
{
    int i;

    if ((!(year % 4) && (year % 100)) || !(year % 400))
        months[1].days = 29;

    year--;
    months[0].start_wday
            = (year * 365 + year / 4 - year / 100 + year / 400 + 1) % 7;

    for (i = 1; i < 12; i++)
        months[i].start_wday =
                (months[i - 1].start_wday + months[i - 1].days) % 7;

    cols = (width + 2) / 22;
    while (12 % cols) cols--;
    gap = cols - 1 ? (width - 20 * cols) / (cols - 1) : 0;
    if (gap > 4) gap = 4;
    lead = (width - (20 + gap) * cols + gap + 1) / 2;
    year++;
}

void print_row(int row)
{
    int c, i, from = row * cols, to = from + cols;
    space(lead);
    for (c = from; c < to; c++)
    {
        i = strlen(months[c].name);
        space((20 - i) / 2);
        printf("%s", months[c].name);
        space(20 - i - (20 - i) / 2 + ((c == to - 1) ? 0 : gap));
    }
    putchar('\n');

    space(lead);
    for (c = from; c < to; c++)
    {
        for (i = 0; i < 7; i++)
            printf("%s%s", wdays[i], i == 6 ? "" : " ");
        if (c < to - 1) space(gap);
        else putchar('\n');
    }

    while (1)
    {
        for (c = from; c < to; c++)
            if (months[c].at < months[c].days) break;
        if (c == to) break;

        space(lead);
        for (c = from; c < to; c++)
        {
            for (i = 0; i < months[c].start_wday; i++) space(3);
            while (i++ < 7 && months[c].at < months[c].days)
            {
                printf("%2d", ++months[c].at);
                if (i < 7 || c < to - 1) putchar(' ');
            }
            while (i++ <= 7 && c < to - 1) space(3);
            if (c < to - 1) space(gap - 1);
            months[c].start_wday = 0;
        }
        putchar('\n');
    }
    putchar('\n');
}

void print_year()
{
    int row;
    char buf[32];
    sprintf(buf, "%d", year);
    space((width - strlen(buf)) / 2);
    printf("%s\n\n", buf);
    for (row = 0; row * cols < 12; row++)
        print_row(row);
}

int main(int c, char **v)
{
    int i, year_set = 0;
    for (i = 1; i < c; i++)
    {
        if (!strcmp(v[i], "-w"))
        {
            if (++i == c || (width = atoi(v[i])) < 20)
                goto bail;
        } else if (!year_set)
        {
            if (!sscanf(v[i], "%d", &year) || year <= 0)
                year = 2016;
            year_set = 1;
        } else
            goto bail;
    }

    init_months();
    print_year();
    return 0;

    bail:
    fprintf(stderr, "bad args\nUsage: %s year [-w width (>= 20)]\n", v[0]);
    exit(1);
}

OUTPUT

                               2016

            January                 Februray                 March        
      Su Mo Tu We Th Fr Sa    Su Mo Tu We Th Fr Sa    Su Mo Tu We Th Fr Sa
                      1  2        1  2  3  4  5  6           1  2  3  4  5
       3  4  5  6  7  8  9     7  8  9 10 11 12 13     6  7  8  9 10 11 12
      10 11 12 13 14 15 16    14 15 16 17 18 19 20    13 14 15 16 17 18 19
      17 18 19 20 21 22 23    21 22 23 24 25 26 27    20 21 22 23 24 25 26
      24 25 26 27 28 29 30    28 29                   27 28 29 30 31 
      31                                              

             April                    May                     June        
      Su Mo Tu We Th Fr Sa    Su Mo Tu We Th Fr Sa    Su Mo Tu We Th Fr Sa
                      1  2     1  2  3  4  5  6  7              1  2  3  4
       3  4  5  6  7  8  9     8  9 10 11 12 13 14     5  6  7  8  9 10 11
      10 11 12 13 14 15 16    15 16 17 18 19 20 21    12 13 14 15 16 17 18
      17