The days are counted
星期一 is Monday, literally "week one", and you carry on to 星期六 for Saturday. Sunday alone escapes the count: 星期天, or 星期日 in writing. So there are six figures to learn and one exception, not seven names.
Zhang laoshi announces the week's classes. Chen Hao has to miss a Friday; Xiaoyu is already thinking about Sunday.
Zhang laoshi
这个星期我们有三节课。
Zhè gè xīngqī wǒmen yǒu sān jié kè.
We have three classes this week.
Xiaoyu
星期几上课?
Xīngqī jǐ shàngkè ?
Which days?
Zhang laoshi
星期一、星期三和星期五,都是下午。
Xīngqīyī, xīngqīsān hé xīngqīwǔ, dōu shì xiàwǔ.
Monday, Wednesday and Friday, all in the afternoon.
Chen Hao
星期五我要去医院,可以请假吗?
Xīngqīwǔ wǒ yào qù yīyuàn, kěyǐ qǐngjià ma ?
On Friday I have to go to the hospital, may I be excused?
Zhang laoshi
可以。星期五的课,你星期六来。
Kěyǐ. Xīngqīwǔ de kè, nǐ xīngqīliù lái.
You may. You can make up Friday's class on Saturday.
Chen Hao
星期六您也在学校吗?
Xīngqīliù nín yě zài xuéxiào ma ?
Are you at the school on Saturdays too?
Zhang laoshi
上午在,下午不在。
Shàngwǔ zài, xiàwǔ bù zài.
In the morning yes, in the afternoon no.
Xiaoyu
那星期天呢?休息吗?
Nà xīngqītiān ne ? Xiūxi ma ?
And Sunday? Is that a day off?
Zhang laoshi
星期天休息,大家都休息。
Xīngqītiān xiūxi, dàjiā dōu xiūxi.
Sunday is a rest day, for everyone.
Xiaoyu
太好了,星期天我想去公园。
Tài hǎo le, xīngqītiān wǒ xiǎng qù gōngyuán.
Wonderful, on Sunday I want to go to the park.
星期一 is Monday, literally "week one", and you carry on to 星期六 for Saturday. Sunday alone escapes the count: 星期天, or 星期日 in writing. So there are six figures to learn and one exception, not seven names.
You ask which day with 星期几, and answer by replacing 几 with the figure: 星期几上课 becomes 星期一上课. That is the ordinary Chinese way of asking, putting the question word exactly where the answer will go. The sentence is not rearranged as it is in English.
早上 early morning, 上午 the morning, 中午 the middle of the day, 下午 the afternoon, 晚上 the evening. These words go before the hour and before the verb: 星期一下午三点, three o'clock on Monday afternoon. Chinese always moves from the wider to the narrower, the opposite of English.