ce mois-ci
I know 'ce mois' means this month but I'm not sure why (-ci) is attached. Could someone explain?

The full sentence is: ce mois-ci, nous vous proposons un test - this month we offer you a test

Merci!

Question

ce mois-ci
I know 'ce mois' means this month but I'm not sure why (-ci) is attached. Could someone explain?

The full sentence is: ce mois-ci, nous vous proposons un test - this month we offer you a test

Merci!

Answer

It's because 'ce' can mean both 'this' and that'.

If you want to distinguish between them, you add 'ci' or 'là'

ce mois-ci  =  this month
ce mois-là  =  that month

Answer

it emphasizes this (ci) or that (là). 
In fact, "ce mois" without "ci" or "là" is seldom used.