Your first impression may be how slowly the Samoans move, and the climate has a lot to do with it. Samoa is closer to the equator than Fiji, Tonga, or Rarotonga, thus it's noticeably hotter and more humid year-round.
May to October (winter) the days are cooled by the southeast trades; winds vary from west to north in the rainy season from November to April (summer). Practically speaking, the seasonal variations are not great, and long periods of sun are common even during the "rainy" months.
Southern Upolu gets more rain than northern, but much of it falls at night. Upolu is generally wetter than Savai'i. The rainfall feeds Samoa's many spectacular waterfalls and supports the luxuriant vegetation.
December to March is hurricane time; ships at Apia should put out to sea at the first warning as the harbor is unsafe when a storm blows out of the north. In recent years, Samoa has suffered an increasing number of devastating hurricanes as the surrounding seas warm up due to climate change.