The Utsavam lasts for ten days. Beginning on the day of Pushya (the 8th asterism) in the month of Kumbham ( February-March), it ends after the Aarattu on the 10th day.Religiously, it is the restoration of divine Chaithanya. Brahmakalasam is preceded by the Utsavam. It is aimed at the purification and energisation of the powers of the deity. It is the last of the long series of rituals of kalasam and at the end, the flag will be hoisted heralding the Utsavam.
Culturally, it consists of various processions, illumination and modest fire-works (this is a specialty of Guruvayur Utsavam that no explosives are used, unlike most of the other Kerala temples). All ten days, the place wears a festive look, streets dressed up with arches, festoons etc., houses freshly thatched and painted.
Every shrine and building is tastefully decorated with lights, plantain trunks, bunches of coconut and arecanuts. Two Gopurams and the bahyankana (outer-courtyard) are elaborately decorated with illuminations and eye-catching electric displays. The lamps, deepasthambams and vilakku are all lightened.
More Info about the festival can be found here.