Questions & Answers about Την Κυριακή θα πάμε στο μουσείο μαζί με την ξαδέρφη μου.
Greek often uses the definite article with days of the week. In this sentence, την Κυριακή is in the accusative and functions like an adverbial time expression meaning on Sunday.
So την Κυριακή = on Sunday (literally the Sunday, but used the way Greek expresses on + day).
Την is literally the (feminine accusative singular). Greek commonly omits a separate word for on with days and uses article + day (accusative) instead.
English: on Sunday
Greek: την Κυριακή
In Greek, days of the week are typically capitalized, similar to English: Κυριακή (Sunday), Δευτέρα (Monday), etc.
Θα is the particle used to form the future in Modern Greek.
πάμε is the 1st person plural present form of πηγαίνω / πάω (we go).
Together, θα πάμε = we will go.