I have to take admission in IGNOU for BCA in July session 2018. When would my first semester examination take place: Dec. 18 or June 19.....

If you have taken the admission for this session, then your exams will take place in December. And, if you haven't taken the admission, then you will be able to take admission for next session in December- January, and exams for this would be conducted in June 19.