April in Reykjavík, Iceland, brings average daily highs of 7°C and overnight lows of 2°C in the 2016–2025 historical record. Warm layers deserve space in the bag. Keep longer outdoor plans easy to adjust, and allow places to stop between walks so that a colder day does not determine the whole itinerary.
In the record, at least 1 mm of rain or snow falls on 15 of 30 days on average, a daily chance of 50%. Give wet-weather alternatives a proper place in the itinerary. Arrange outdoor plans so you can move them between days, and pack a rain layer rather than relying on a completely dry run of weather.
The calculated day length, sunrise to sunset, averages about 15 hours. Sunshine averages 9 hours a day. The longer daylight window gives you more choice about when to be outside; it does not mean every part of the day will be sunny. Sunshine is a separate measure, and those hours need not arrive in one uninterrupted stretch.
For nature trips or walks, keep the longest outings movable and check route conditions locally; a monthly climate average cannot establish whether a particular path is accessible. If the coast is part of your plan, check water conditions separately. Air temperature and sunshine do not establish swimming conditions, surf quality or diving visibility.
May has an average daily high of 10°C, 3°C higher than April. Its historical rain-day chance is 58%, compared with 50% this month. If your dates are flexible, compare those trade-offs with the kind of days you want. Neither set of averages predicts the conditions of one particular holiday.