七

English builds 17 as seven-ten, but Chinese builds it as ten-seven: 十七. The 十 sits in front for every teen, from 十一 up to 十九. Once you pass 20, 十 moves into the middle: 二十七 is two tens plus a seven. That means 五十七 is 57, and 十五七 is gibberish born from stretching the teen pattern too far. Check where 十 sits and you instantly know if it's a teen or a ten. “六七八九十， 昨天星期四； 十四是十四， 四十是四十。” (Six, seven, eight, nine, ten — yesterday was Thursday. Fourteen is fourteen, and forty is forty.) ── [《一二三（2）》](/read/counting-days-rhymes#s2)

You get 星期一 through 星期六, and then the counting just stops. The seventh day is 星期天 or 星期日, using the words for sky and sun instead of a number. Say 星期七 and people will still get it, but it lands like a dropped stitch in a sweater. Casual speech also gives you 周日 and 周天.

An old Beijing habit turned 七 and 八 into second tone right before a fourth tone, so 七个 came out as qí gè. Modern dictionaries and the official pronunciation list drop that flip and keep 七 as plain first tone. You will still catch qí gè from older northern speakers, and it sounds perfectly local. Say qī gè, and nobody will ever correct you.