In Seoul the spring forecast comes with a fine-dust reading, and people check it the way Londoners check for rain. In April you should too. The cherry blossom is excellent, peaking in early April in most years, with highs around 18°C. But March and April are usually the worst months for yellow dust blown in from China and Mongolia. Bad spells tend to last only a few days. Put an air-quality app on your phone and a KF94 mask, sold in any convenience store, in your pocket. Save the climb up Namsan for a clear day.
In April the walking paths along Seoul's streams come lined with cherries. Anyangcheon, in the west, has the longest cherry path in the city, thickest through Mokdong. Jungnangcheon, in the north-east, has a blossom tunnel of its own and far fewer phones held aloft. On Namsan, start near the National Theatre of Korea and climb through the trees; some of those paths are oddly quiet for the middle of the capital.
Yunjung-ro, the road behind the National Assembly on Yeouido, is the city's best-known blossom street, where the King cherry trees meet overhead. Seokchon Lake in Songpa is prettier, with about a thousand trees ringing the water below Lotte World Tower, and every bit as crowded. Both hold festivals in early April, on dates announced each spring. Walk Yunjung-ro at breakfast time on a weekday, or not at all.
If you end up there on a festival evening anyway, do as Seoul does and drop down to Yeouido Hangang Park, where fried chicken arrives by scooter at numbered delivery zones near the entrances. Give the restaurant the zone number, not your picnic spot, and wait by the sign.