# Number of days between month1/day1/year1 and month2/day2/year2: 
# days(year2,month2,day2) - days(year1,month1,day1); 

days := proc(y,m,d)    # y = (4-digit) year, m = month, d = day
     local f,y1,m1;
         f := floor(m/12+3/4);
         m1 := m+9-12*f;
         y1 := y-1+f;
         floor(365.25*y1)-floor(y1/100)+floor(y1/400)+floor(30.6*m1+.4 )+d
     end;

wd := (y,m,d) -> irem(days(y,m,d)+2,7);

# Weekday of month/day/year: weekday(year,month,day); 

weekday := (y,m,d) -> op(wd(y,m,d)+1,['Sun','M','Tu','W','Th','F','Sat']);

# Examples: 
# > weekday(1997,12,25);
# 
#                                        Th
# 
# > days(1997,12,25) - days(1912,4,14);
# 
#                                      31301