'Summery' describes something that feels like summer or suits summer. It is not a fact about the calendar — it is a feeling. A thin yellow dress is summery. A salad with cold noodles and lots of lemon is summery. A bright, light song is summery. Crucially, you can have a summery day in March, if the sun comes out and it is unexpectedly warm; a British person will say 'It's quite summery today' with real pleasure, because it is a surprise. So the word carries lightness, brightness, warmth, colour, and often a bit of cheerfulness. It is used mostly of weather, clothes, food and drink, colours, and moods or atmospheres. Compare it with 'summer' used as a describing word before a noun: a 'summer dress' is a dress for the summer season, a plain statement; a 'summery dress' is a dress that looks and feels like summer, which could be worn in any season. The '-y' ending is doing the same job it does in 'cloudy' or 'sleepy' — 'having the quality of'. The opposite pairs are 'wintry' (note: not 'wintery' in careful spelling) and 'springlike'.