Vietnamvisa exemption

Hello:
 
I am still planning the last details about my entry date in Vietnam, but my stay will most likely be longer than 15 days.
 
I am considering entering Vietnam from November 28 and I already have the departure ticket to Spain confirmed for December 20.
 
In the letter of invitation to Vietnam I have indicated, as an entry approach to the country, on December 1st.
 
My question is this: this date (December 1) is final or could enter the country a few days before?
 
Thank you very much.

 Leyre Fernández González.