Language: C

d,s[]={0,31,60,91,121,152,182,213,244,274,305,335};main(m){char  [9][9]={"Thurs","Fri","Satur","Sun","Mon","Tues","Wednes"};while(1){scanf("%d%d",&m,&d);if(!m)exit(0);d+=s[m-1];printf("%sday\n",a[(!d%7)?6:(d%7-1)]);}}

Compiler output:
main.c:1:1: warning: data definition has no type or storage class
    1 | d,s[]={0,31,60,91,121,152,182,213,244,274,305,335};main(m){char  [9][9]={"Thurs","Fri","Satur","Sun","Mon","Tues","Wednes"};while(1){scanf("%d%d",&m,&d);if(!m)exit(0);d+=s[m-1];printf("%sday\n",a[(!d%7)?6:(d%7-1)]);}}
      | ^
main.c:1:1: error: type defaults to 'int' in declaration of 'd' [-Wimplicit-int]
main.c:1:3: error: type defaults to 'int' in declaration of 's' [-Wimplicit-int]
    1 | d,s[]={0,31,60,91,121,152,182,213,244,274,305,335};main(m){char  [9][9]={"Thurs","Fri","Satur","Sun","Mon","Tues","Wednes"};while(1){scanf("%d%d",&m,&d);if(!m)exit(0);d+=s[m-1];printf("%sday\n",a[(!d%7)?6:(d%7-1)]);}}
      |   ^
main.c:1:52: error: return type defaults to 'int' [-Wimplicit-int]
    1 | d,s[]={0,31,60,91,121,152,182,213,244,274,305,335};main(m){char  [9][9]={"Thurs","Fri","Satur","Sun","Mon","Tues","Wednes"};while(1){scanf("%d%d",&m,&d);if(!m)exit(0);d+=s[m-1];printf("%sday\n",a[(!d%7)?6:(d%7-1)]);}}
      |                                                    ^~~~
main.c: In function 'main':
main.c:1:52: error: type of 'm' defaults to 'int' [-Wimplicit-int]
main.c:1:66: error: expected identifier or '(' before '[' token
    1 | d,s[]={0,31,60,91,121,152,182,213,244,274,305,335};main(m){char  [9][9]={"Thurs","Fri","Satur","Sun","Mon","Tues","Wednes"};while(1){scanf("%d%d",&m,&d);if(!m)exit(0);d+=s[m-1];printf("%sday\n",a[(!d%7)?6:(d%7-1)]);}}
      |                                                                  ^
main.c:1:134: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
    1 | d,s[]={0,31,60,91,121,152,182,213,244,274,305,335};main(m){char  [9][9]={"Thurs","Fri","Satur","Sun","Mon","Tues","Wednes"};while(1){scanf("%d%d",&m,&d);if(!m)exit(0);d+=s[m-1];printf("%sday\n",a[(!d%7)?6:(d%7-1)]);}}
      |                                                                                                                                      ^~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | d,s[]={0,31,60,91,121,152,182,213,244,274,305,335};main(m){char  [9][9]={"Thurs","Fri","Satur","Sun","Mon","Tues","Wednes"};while(1){scanf("%d%d",&m,&d);if(!m)exit(0);d+=s[m-1];printf("%sday\n",a[(!d%7)?6:(d%7-1)]);}}
main.c:1:134: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
    1 | d,s[]={0,31,60,91,121,152,182,213,244,274,305,335};main(m){char  [9][9]={"Thurs","Fri","Satur","Sun","Mon","Tues","Wednes"};while(1){scanf("%d%d",&m,&d);if(!m)exit(0);d+=s[m-1];printf("%sday\n",a[(!d%7)?6:(d%7-1)]);}}
      |                                                                                                                                      ^~~~~
main.c:1:134: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:1:160: error: implicit declaration of function 'exit' [-Wimplicit-function-declaration]
    1 | d,s[]={0,31,60,91,121,152,182,213,244,274,305,335};main(m){char  [9][9]={"Thurs","Fri","Satur","Sun","Mon","Tues","Wednes"};while(1){scanf("%d%d",&m,&d);if(!m)exit(0);d+=s[m-1];printf("%sday\n",a[(!d%7)?6:(d%7-1)]);}}
      |                                                                                                                                                                ^~~~
main.c:1:1: note: include '<stdlib.h>' or provide a declaration of 'exit'
  +++ |+#include <stdlib.h>
    1 | d,s[]={0,31,60,91,121,152,182,213,244,274,305,335};main(m){char  [9][9]={"Thurs","Fri","Satur","Sun","Mon","Tues","Wednes"};while(1){scanf("%d%d",&m,&d);if(!m)exit(0);d+=s[m-1];printf("%sday\n",a[(!d%7)?6:(d%7-1)]);}}
main.c:1:160: warning: incompatible implicit declaration of built-in function 'exit' [-Wbuiltin-declaration-mismatch]
    1 | d,s[]={0,31,60,91,121,152,182,213,244,274,305,335};main(m){char  [9][9]={"Thurs","Fri","Satur","Sun","Mon","Tues","Wednes"};while(1){scanf("%d%d",&m,&d);if(!m)exit(0);d+=s[m-1];printf("%sday\n",a[(!d%7)?6:(d%7-1)]);}}
      |                                                                                                                                                                ^~~~
main.c:1:160: note: include '<stdlib.h>' or provide a declaration of 'exit'
main.c:1:178: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
    1 | d,s[]={0,31,60,91,121,152,182,213,244,274,305,335};main(m){char  [9][9]={"Thurs","Fri","Satur","Sun","Mon","Tues","Wednes"};while(1){scanf("%d%d",&m,&d);if(!m)exit(0);d+=s[m-1];printf("%sday\n",a[(!d%7)?6:(d%7-1)]);}}
      |                                                                                                                                                                                  ^~~~~~
main.c:1:178: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:1:178: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:1:178: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:1:195: error: 'a' undeclared (first use in this function)
    1 | d,s[]={0,31,60,91,121,152,182,213,244,274,305,335};main(m){char  [9][9]={"Thurs","Fri","Satur","Sun","Mon","Tues","Wednes"};while(1){scanf("%d%d",&m,&d);if(!m)exit(0);d+=s[m-1];printf("%sday\n",a[(!d%7)?6:(d%7-1)]);}}
      |                                                                                                                                                                                                   ^
main.c:1:195: note: each undeclared identifier is reported only once for each function it appears in