It's beyond serious if you mess up and end up with an unplanned pregnancy.
Call your ob/gyn's office and ask them. We're not doctors. And I find the question confusing, myself.
Getting off your schedule with birth control pills is asking for trouble. Making assumptions about what day you'll start your period is also asking for trouble (it can change from month to month--even the most predictable period cycle can change so don't make assumptions about that either).
Is there a reason you're not asking your ob/gyn's office directly rather than asking online? Does that office not want to answer questions by phone? All my gynecologists' offices were great about answering questions, but if yours is not, you might need to go in there in person before your trip, so check with them today.