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<div class="day">
  <div class="dayimg"></div>
</div>
<div class="day">
  <div class="dayimg"></div>
</div>
<div class="day">
  <div class="dayimg"></div>
</div>
<div class="day">
  <div class="dayimg"></div>
</div>
<div class="day">
  <div class="dayimg"></div>
</div>

const days = document.querySelectorAll('.day')

days.forEach((el, i) => {
  el.querySelector('.dayimg').textContent = i
})

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