This is the 143rd most frequent Icelandic word.
fyrstu
"Fyrstu" means "first" (plural or feminine singular definite form) in English.
Here, 'fyrstu' is used in the feminine singular form as an adjective to describe 'vikuna' (the week). It means 'first' in this context.
Eftir fyrstu vikuna í skólanum var ég mjög spenntur.
After the first week of school, I was very excited.
In this sentence, 'fyrstu' is used as an attributive adjective in the plural form to modify the noun 'skiptin' (times). It emphasizes the initial occurrences.
Hún klæddist sínum fallega kjól í fyrstu skiptin.
She wore her beautiful dress for the first times.
In this usage, 'fyrstu' modifies the plural noun 'gestirnir' (the guests), describing them as the initial individuals who arrived.