Thanks for contacting ask extension with your question about the proper time to plant Dahlias. 
The best time to plant Dahlias outside is when the soil temperature is around 60º or warmer consistently for a number of days. If the soil is cooler or wet, it’s better to wait as cool/wet conditions increase the risk of tuber rot. 

To measure the temperature, use a soil thermometer 4–6" deep. 
Or to get an approximation you can use the data from the Hysop Weather Station (HWS): https://agsci.oregonstate.edu/hyslop-weather-station
According to Hysop data, the soil at 4" deep is just now reading lows in the 50s and highs just touching 60º. Remember, conditions may be different depending on your location and elevation. 
I recommend you hold off until the low temperatures are closer to the high 50s or 60s.
When you do decide to plant:
Plant tubers about 4–6 inches deep, laid horizontally with the eye up; stake tall varieties at planting to avoid damaging tubers/roots later. 
After planting: soil should be slightly moist, but don’t water again until you see the first two leaves (this helps prevent rotting).

Here's a great OSU publication you may find helpful. "Dahlias in Oregon, Planting for a Parade of Colors in Fall: https://extension.oregonstate.edu/catalog/fs-95-dahlias-oregon
Happy gardening!