Hi Sheila,  That is a tough question to answer due to unknown weather conditions. To be on the safe side you can certainly cut back your dahlias now, dig them, dry them for a few days in a covered area and then pack them away for the winter in a cool dry spot.  However it is most likely that by early December the light frost will have yellowed the foliage of your dahlias as nature intended giving the plants more time to make energy for the underground roots. Then when you return from Europe (hope you have a lovely time by the way) your dahlias will be ready to dig and store. The tubers will be fine if there is a frost while you are gone. It is a hard freeze or a winter's worth of rain that can rot and destroy dahlias tubers in the ground.  I grow dahlias in raised beds and because of the good drainage I do not need to dig and store them in the winter. Your soil may hold too much moisture or we could get an early freeze and your investment of time planting 100 dahlias is pretty high.  The weather this next week will be sunny and mild so digging and storing your dahlia tubers before you leave gets my vote. Keep growing, Marianne Binetti