1. The next meeting is at 9:00 a.m. on April 3.
2. The next meeting is at 9:00 a.m., April 3.
3. The next meeting is at 9:00 a.m. April 3.
4. The next meeting is 9:00 a.m. on April 3.
5. The next meeting is on April 3
at 9:00 a.m.
( In formal writing, which one should I write? Thank you for your help.)