It is usually not necessary to have 在 before a time phrase; however, 在 is used when a word such as 要, 想, 会 or 将 precedes the time phrase. 要, 想, 会 or 将 is used to indicate that the event or action will take place in the future.
Note that 于 (yú) can replace 在 in more formal situations.
我下星期五要在法院结婚；我们将在星期六请客。
Wǒ xià xīng qī wǔ yào zài fǎ yuàn jié hūn ；wǒ men jiāng zài xīng qī liù qǐng kè 。
I am getting married next Friday at the court house; we will have a reception on Saturday.
(Because of 将, 在 is used before星期六.)
(Compare: 我们下星期五结婚，星期六请客。)
老师会在明天上课的时候告诉我们下次考试的重点。
Lǎo shī huì zài míng tiān shàng kè de shí hòu gào sù wǒ men xià cì kǎo shì de zhòng diǎn 。
The teacher will tell us the focus of the next test during class time tomorrow.
(Compare: 明天上课的时候，老师会告诉我们下次考试的重点。)
[1] Yip, P. C., Rimmington, D., Xiaoming, Z., & Henson, R. (2009). Basic Chinese: a grammar and workbook. Taylor & Francis.
