MrsE & Genni - don't worry too much. Th cb ones are designed to be used in the morning and they build up a knowledge of your hormone levels. By using them twice a day it could give you a false 'peak reading'. My advice would be to use them as the instructions say. Once you have a peak, dtd that day and for a few days after. The peak indicates that you are likely to ov in the coming 24-36 hours. Maybe use ic opks in the afternoon alongside the cb ones.
When I got my bfp, I had a peak on cd15, but my temps show ov at cd 18! so def keep going. We dtd everyday from cd15-18.
Bella - i think you should continue ttc. It doesn't guarantee you'll def fall this cycle. And maybe if you did fall dm&d might postpone the holiday? The thing is, you could keep putting it off, if you let other people determine when you start again.