This is the 527th most frequent Persian word.
پنجشنبه
Thursday
The word "پنجشنبه" (Thursday) is used here as a specific time reference referring to the day of the trip.
ما پنجشنبه یک سفر کوتاه خواهیم داشت.
We will have a short trip on Thursday.
Here, "پنجشنبهها" (Thursdays) is in its plural form to describe a recurring occurrence related to the day.
پنجشنبهها روزهای مورد علاقه من هستند.
Thursdays are my favorite days.
In this case, "پنجشنبه" (Thursday) is used with the postfix "قبل" (last) to specify the Thursday of the previous week.