There's no single “best” month for North Macedonia — there's a best month for what you want to do. Lake swimming, mountain skiing, wine harvest, and canyon hiking each have their own ideal window, and the country is small enough that the answer changes depending on which corner you're planning around.
March–April: shoulder spring
Mild, unpredictable weather. Good for Skopje sightseeing and early hiking at lower elevations, but Lake Ohrid is still too cold to swim and mountain resorts are winding down.
May–June: the sweet spot
Widely considered the best all-round window. Warm enough for Lake Ohrid, green and comfortable for hiking and Matka Canyon, and before the July–August price and crowd peak.
July–August: peak summer
Hottest, busiest, and most expensive across the lake and coastal-feeling towns. Best for swimming and festival season (Ohrid Summer Festival), but book accommodation well ahead.
September: the other sweet spot
Many locals consider September even better than May — warm lake water, thinning crowds, and the start of the Tikveš wine harvest.
October–November: quiet autumn
Cooling fast, especially in the mountains. Good for photography and Old Town wandering, less good for lake swimming or long hikes at altitude.
December–February: ski season
The only real draw for this stretch is skiing at Mavrovo and Popova Šapka — otherwise, most of the country is quiet, cold, and better visited for its thermal spas in Strumica than for outdoor sightseeing.