"Субота - шостий день."

Translation: Saturday is the sixth day.

[Maironlucas]

Is Saturday the seventh day?

[deactivated user]

This is different in different countries. Ukrainians generally start counting days of the week from Monday.

[flameofgrace]

I was thinking the same thing. Especially with Saturday being associated with Sabbath.

[John686606]

Does Шостий always take the male form as I would assume субота is female?

[RianFinn]

шостий is the adjectival form of the number six which are called ordinal numbers.
Because they describe nouns, they match in gender (m/f/n), number (sing/pl), and case (nom/gen/etc).
In this case, "шостий" is describing день (not Saturday) which is why it's in the masculine singular nominative.