It's getting brighter in the day, and cooler and crisper in the nights in and around Machu Picchu this month. Cusco area - highs around 66° F (19° C) and lows around 37° F (3° C). The temperature variation between average highs and average lows is 9° F (4° C) greater than it was in January, increasing the need to pack both for the hot weather and the cold.
At Machu Picchu, bring sun block to protect against the sun, which is increasingly strong during the day now (the site also has scant shade). Walking around these Inca ruins in the increasing crowds you may be favoring shorts and t-shirts, but at night or early morning you will certainly want that warm fleece and under-layer to abate the chill.
Despite the clearer weather, Machu Picchu remains regularly shrouded in cloud in early morning, so view points on the site such as the Sun Gate or Wayna Picchu may not yield those desired panoramic views first thing.
Crowds & Costs
Better weather for doing outdoors activities and a spate of interesting festivals, plus the swing from shoulder season towards high season for North American, European and Australian/New Zealand visitors, makes for many more crowds than most months of the year. After June through August, May is probably the next-most popular time to visit the Sacred Valley. But early-to-mid-May is still very much shoulder season, and so crowds are still far short of what they become in June through August. So you may, especially in the early part of the month, still get lower rates for hotels and activities than you would in high season.
Where to Go
May is an outdoor month, for sure, and many of the other Inca sites in the Sacred Valley outside Machu Picchu such as those at Pisac, Ollantaytambo or Moray are still fairly crowd-free in May. But this is also a month when the communities hereabouts begin to come to life with festivities. Cusco, ever a centre of activity, has its Vigil of the Crosses, but the towns of Pisac and Ollantaytambo have their own festivals this month which make them appealing to visit.
What to Do
As the chance of rain dwindles still further compared to previous months, hiking is more and more popular in the Sacred Valley, and hikes up to some of the other big Inca ruins such as Moray near Maras are appealing right now.
May is a brilliant season for wildlife-lovers. The Sacred Valley still looks emerald-green and plants and flowers are thriving at the moment after wet season rains.
If spotting mammals and birds is more your thing, good news. If you are incorporating a visit to the much-lower jungle before or after Machu Picchu (the Inca Jungle Trek, for example) falling water levels mean animal sightings on the riverbanks are increasing, and birds are more commonly seen from canopy viewing platforms because they like warming themselves in the sun. Falling water levels mean this is the last month for navigating many jungle tributaries by canoe and grabbing the best sightings of monkeys: these tributaries will be dried up from next month, so, once May is over, monkey sightings go down.
This is one of the best months (along with June) for rafting the Río Apurímac, also on other rivers the rafting is waning as dry season lowers water levels.
May is one of the better months for festival-goers in the Machu Picchu area too.
May might not have next month's blockbuster Inti Raymi (Inca Festival of the Sun) celebrations, but it does boast several lesser-known festivities, especially in the smaller towns in the Sacred Valley.
Vigil of the crucifixes: On May 2 or May 3 a crucifix vigil is held atop all hills in the Cusco area with crosses atop them (quite a few!).
San Isidro Labrador: Festivities kick off in Pisac on May 15 to celebrate this saint. Costumed revellers and cows decorated specially for the occasion make a procession, and there is music, dancing and feasting.
Q'oyoroti: This celebration, the lesser-known counterpart to Inti Raymi, is the traditional Andean honoring of the Inca Festival of the Sun (without the international festival-goers that Inti Raymi attracts. It is held at Ausangate on the Tuesday before Corpus Christi (in late May or early June).
Señor de Choquechilca: Ollantaytambo's most important festival occurs in late May, remembering the moment when a wooden cross appeared next to the town's Inca Bridge. Locally, this event was considered a miracle and is celebrated accordingly, with lots of music and dancing, plus processions.