Santa Marta is hot, year ‘round. It’s Caribbean. However, the evenings are bearable because of the breeze that is swept off the sea late afternoon.
Rainy season (and the season that is most difficult to hike The Lost City) is in February – the rainiest month. The driest month is October.
Colombians take their vacations, usually, in December and January, June and July – so Santa Marta will be much busier, and more expensive, during those times of year.