I’m planning to leave on the seventeenth (=the 17th day of the month).seventeensev‧en‧teen /ˌsevənˈtiːn◂/ ●●● S3 W3 number 
a group of seventeen American military officers I left home when I was seventeen (=17 years old).—seventeenth adjective, pronoun: in the seventeenth century her seventeenth birthday I’m planning to leave on the seventeenth (=the 17th day of the month).
WORD ORIGINseventeen
Origin:
Old English seofontiene, from seofon ‘seven’ + tien ‘ten’EXAMPLES FROM THE CORPUS
- He was seventeen when he won the Open championship belt for the first time.
- When she was seventeen, the family moved to Puerto Rico.
COLLOCATIONS FROM THE ENTRY I’m planning to leave on the seventeenth (=the 17th day of the month).
the number 17: I’m planning to leave on the seventeenth (=the 17th day of the month). a group of seventeen American military officers I left home when I was seventeen (=17 years old).—seventeenth adjective, pronoun: in the seventeenth century her seventeenth birthday I’m planning to leave on the seventeenth (=the 17th day of the month).