祝贺 大公司的OFFER! Accept it and go to work in there!

Congrate for the better offer!

Accept the Christmas Gift and be very happy with it.
No any problem. There is NOTHING to worry.
Nobody will care this one month and nobody will know this one month.

Dear Small_Company_Manager,
I appreciate (very much) the nice time worked with you.
I just got a offer which I can't reject, (Mr. Santa need more people to help to prepare next years Christmas gift.), therefore, I will not come back after Christmas.

Thanks!