The best time to safely visit the place would be from the beginning of May till the end of September. The snow should be melted by this time and the access road should be safer to move on. If you are planning to walk from Smrdliva Voda to the range it will be a two day trip - the distance is almost 20 km one way. Check the weather forecast before you go. The weather can change very quickly in the mountains.
The Kozuf Mountain is located in the southern part of Macedonia (or FYROM). The best access to the base of the mountain is from the town of Gevgelija on the east. There is a road that leads from Gevgelija to Smrdliva Voda (Stinking Water in English). Smrdliva voda is the last village before the Kozuf mountain. There is a source of mineral water and a small church on the rock. A nice place to visit indeed. The road up the mountain starts from here but recently the road was damaged by landslides. When I visited it in May 2017 it was still possible to go up in normal car but with great caution. There were stones and mud on the road. You can get to the Ski Kozuf centre by this road. It lies near the mountain but is currently out of business (don't know the reason). There is a road and a path to the col where is the upper station of the ski lift. From there go south along a smaller ski lift and you will get to the top of Zelen Breg. Zelen Breg is the highest peak of Kozuf with 2172 meters. The views south are great. The peak itself is near the Macedonian-Greek border.