Denver is a central American city at the foot of The Rocky Mountains. There is very little rain and most days are sunny and warm (up to 300 of them typically). There are some chances of snow in winter but it rarely lies for very long.
The average summer temperature is 23 °C whilst the average winter temperature is 0°C.
For the most reasonable weather visit in late spring and summer but any time of year is applicable.
Dress and plan accordingly.