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seachdainweekfeminine noun
the
an t-seachdain
after a preposition
an t-seachdain
of the
na seachdaine
plural
seachdainean
the (plural)
na seachdainean

Every form but the headword, the genitive and the plural is worked out by the spelling rules in the grammar guide, so what you see here is what the guide teaches. Awaiting a fluent speaker’s review.