Celebración vs. festejo vs. fiesta

The words 'celebración', 'festejo', and 'fiesta' in Spanish all refer to celebrations, but they are used in slightly different contexts with some nuances.

Celebración

'Celebración' corresponds directly to the English word 'celebration'. It refers to a social gathering or event that is being held to celebrate a particular occasion, such as a birthday, anniversary, holiday, etc.
La celebración del cumpleaños de Juan fue muy divertida.
(Juan's birthday celebration was very fun.)
Cada año, se lleva una gran celebración por el Día Nacional.
(Every year, there is a big celebration for National Day.)

Festejo

'Festejo' is also a celebration, particularly an organized series of special events and activities created to honor an occasion or person. Compared to 'celebración', it emphasizes the action of celebrating more than the event itself.
El festejo por la victoria del equipo de futbol duró toda la noche.
(The celebration for the football team's victory lasted all night.)
Después del festejo de su ascenso, se sentía muy contento.
(After the celebration of his promotion, he felt very happy.)

Fiesta

'Fiesta' can also mean 'party', in addition to a celebration. This term is often used for informal gatherings involving music, dance, food and drinks.
A Ana le encanta organizar fiestas en su casa.
(Ana loves throwing parties at her house.)
La fiesta de fin de año fue inolvidable.
(The end-of-year party was unforgettable.)

Summary

In conclusion, while 'celebración', 'festejo', and 'fiesta' all reference celebrations in Spanish, their usage varies subtly based on the context and nature of occasion. 'Celebracion' aligns most closely with the English 'celebration', while 'festejo' emphasizes the action of celebrating. Lastly',fiesta' often denotes informal party-like gatherings.