30days notice is required to cancel. If your minimum term expires on 21st December, then you should be giving that notice on or around November 21st.
It's quite possible that, by requesting renegotiation of your package 2months before your minimum-term expires, VM retentions know that currently you're not in a position to cancel and thus that's potentially reflected in whatever offer they've given you.
Do your homework on alternative providers for your services, and ensure you're familiar with install lead times first - then bear that in mind when you call on or around Nov 21st.