SR20 pilot reported failure to utilize proper frequency resulted in an NMAC with opposite direction landing traffic.

Narrative

After takeoff and during initial climb we encountered a traffic conflict with an approaching aircraft that required both of us to sidestep to evade each other. Our departure from runway XX at ZZZ was uneventful; however; during initial climb an aircraft surprised us attempting to land on the opposite runway XY. We transmitted a caution and observed the other aircraft sidestepping to their right to avoid conflict. At a safe altitude during crosswind I realized that our frequency for CTAF was incorrect; likely due to an inadvertent push of the frequency switch button during preflight and/or before takeoff checklist. After switching to the correct frequency; we broadcast our realization and heard the other aircraft attempt to contact the airport manager. We heard the aircraft execute a go-around and enter the downwind before we departed the traffic pattern.The proximate cause was an incorrect radio frequency in our plane that was not discovered until the incident occurred.Traffic at the airport was mostly idle due to an on-airport open house that had reduced activity per NOTAM. We believed that we were the only active aircraft due to [event]. So; it was not unexpected to have heard no radio calls in the area. Also; the visual scan for traffic did not reveal the incident aircraft on downwind as we entered the runway for takeoff.Corrective actions would be to re-verify the correct CTAF frequency prior to entering the departure runway; both visually from the avionics and with a radio-check call.