every summer
Straightforward temporal noun meaning 'each summer' or 'every summer'. Used adverbially with or without a particle.
毎夏、家族で海に行きます。
Every summer, we go to the beach as a family.
この町では毎夏花火大会が開かれる。
A fireworks festival is held every summer in this town.
Compound of 毎 (every) and 夏 (summer). The reading まいなつ follows the standard on'yomi pattern for such compounds.