Thanh Hoa Province has a monsoon climate with a long hot season and distinct wet and dry periods that vary by altitude.
October to April is generally the best time to visit. Temperatures are mild (20–28°C in the lowlands), rain is infrequent, and the roads to Pu Luong are passable. Sam Son Beach is quiet in this season — fine for a day trip but lacking the summer atmosphere.
May to August brings heat and humidity to the coast and lowlands, with temperatures regularly exceeding 35°C. Sam Son Beach fills up with Vietnamese holiday-makers in July and August — the town has genuine summer resort energy if that appeals, but book accommodation in advance. Pu Luong is wet, muddy, and leech-heavy during the peak monsoon months.
September is transitional: the monsoon eases but typhoons can occasionally affect the northern coast.
For trekking in Pu Luong, October to December and March to April are the best windows — dry enough for comfortable walking, cool enough for highland nights.