How to stay calm and focused while coordinating a special event?

Enjoy doing every bit of it, right from the planning, coordinating and executing stages.
Teamwork counts; Coordinate with every one involved in the team and make sure each of them are aware of their responsibilities.
Keep a plan B ready incase of uncertainties.
Make couple of rounds to the venue before the event to check if everything is in order, run through the agenda and time schedule time and again till everything is double checked and finalized.
Hurry and worry will only make things worst, keep yourself and your team cool and motivated by treating with a nice snack break and motivating comments by appreciating each of their efforts.
Be positive of what you are doing and definitely things will turn out well even in the worst of situations.

Do not admit the liability ask for a help

Plan in advance and communicate with your team, suppliers, clients on constant basis.

Make sure everybody in your team knows what they are doing and are aware of the deadlines.

Keep a list of To-Do things and cross them off once they are fulfilled.

Don't try to do everything by yourself - trust your team. Your main task is to coordinate and manage.

If something goes wrong, don't panic and blame yourself or your team. Instead use precious time to fix the problem.