Daylight hours – the sun shines an awful long time during the summer months. May, June has over 17 hours of daylight (sunrise 04:55 - sunset 22:11hrs), July and August have around 14 hours of daylight (sunrise 06.30 - sunset 20.21hrs) so are the months with maximum daylight. December sunrise is around 8:30am and sunset is 4pm - 7 hours of daylight.
March is when the first visitors arrive. The vegetation has no leaves, the days are cold, but lengthening.
April is the month you really feel summer is on the way. Towards the middle of the month, some of the bushes start to turn green. The early morning birds songs are wonderful, as this is when the birds are pairing up and breading.
May is when all the leaves appear back on the vegetation, how early this is, depends on how hard the winter has been. The countryside is arguably at its best, at the end of the month. The lovely fresh green of new foliage. The dawn chorus, when all the birds call and attract mates for this year's breading is also special. However it's also has a number of local public holiday weekends, which can make accommodation choice limited unless you book well ahead.
June is a wonderful month, the days are long. The longest day is the 21st of June (sunrise 4.30am - sunset 10.30am) - this a wonderful month to tour. The local Scottish school holiday do not start until the end of the month so it is not so busy.
July is when the local Scottish schools are on holiday, to be joined later in the month by the English and Welsh schools. This means accommodation is harder to find, and commands premium prices.
July and August are the peak tourist months. A major event in August is the Blair Castle Horse Trials, so accommodation can be difficult to find at late notice.
September sees all the schools go back, so it is a good time to holiday. The year's last Highland games takes place in the second weekend of the month in Pitlochry which are always very popular. Accommodation is more readily availability, except for the games weekend when rooms sell out.
October is when the autumn colours are at their best, and of course the Enchanted Forest takes place at Faskally Woods. This is almost the end of the tourist season. But it is a good month to see the salmon run up the rivers to spawn and hear the red deer rutting. The last hurrah before winter, then most of the tourists disappear.
No matter the time of year, everyone's opinion for the best time to Scotland differs.