Life in Denver can be difficult in April.
Main focus: Outside water systems
Hoses: Warm days in the 70's, nights dip down into the 30’s inspires gardening of all sorts. If the forecast is below 32 F, disconnect the outside hose. Leaving the hose on a frost free hose faucet (bib) in cold weather will crack this expensive valve.
Resist the temptation to turn on sprinklers early in spring. Make sure the drain valves are closed prior to turning on the water.
There are valves on the Vacuum Breaker (shown below) and possibly a drain valve near your irrigation water turn on valve.
The picture shows the valve turned to off.
Study up on the Sprinkler Controller. Now is a good time to consider drip irrigation for targeted watering.
If your Sprinkler has ball valves: Move the valve handle slowly. Opening the valve fast puts a water hammer on weak joints.
Many sprinkler systems are plumbed with PVC (plastic) fittings that may get brittle.