Simple way is, locate card opening date in CIBIL: Complete your closure request before that day (Note: Complete not request).
Ex: 15 Feb, 2022 your card opening date according to CIBIL.
In this case, you have to place closure request before 14th Feb, 2022 but
Note: They'll take some time to process your closure request and offer any retention etc. based on your profile and that can take up-to 7 days.
So, have to place closure request before 5th Feb, 2022 considering non-working days.