Here is a great article by broadcast engineer Paul Thurst, KH2R.
He explains two long-range VHF propagation phenomena: Tropospheric ducting and sporadic E.
Tropospheric ducting is fairly common in the early summer months. I've personally heard it as early as mid-May, and throughout June. A good way to tell if there is ducting happening is to tune to an FM radio channel with no station on it. You'll end up listening to a lot of static, but stations from far away will come in crystal clear once propagation picks up!
Of course, you can also listen or call CQ on VHF calling frequencies for long distance activity:
6 meters - 50.125 USB, 52.525 FM
2 meters - 144.200 USB, 146.520 FM
1.25 meters - 222.100 USB, ? FM
70 centimeters - 432.100 USB, 446.000 FM
Happy DXing!
- KC1DPM