Look at what day of the month your statement closing date is. Now put December in front of it. For me, since my closing date is the 3rd, my $6000 annual limit for 6%cb on groceries needs to post by December 3rd and then the calendar resets. This is only an important detail if you are trying to maximize the 6% each year. If not, then just use the card as your go-to card for groceries and don't worry about the $6000 limit (unless you're getting close to reaching the limit, since then it drops to 1%). This year, I'm not even close to reaching 6,000 on regular grocery spending (I shop at Costco too), so I'll be buying a lot of gift cards when it gets closer to Dec. 3rd.
Make sure you check out your AMEX Offers too. Sometimes there are really great ones, and they are not always the same on all AMEX cards. I'm an AU on DH's AMEX BCP and I can add offers to my card as well as his.