on /at

[ziawj2]

We are going to make a spring excursion to the Fragrant Hill in the suburbs of Beijing at April 5-6.
or
on April 5 and 6.
or
from April 5 to 6.
which is acceptable to native speakers? And do you have any other suggestions?

[emsr2d2]

We are going to make a spring excursion to the Fragrant Hill in the suburbs of Beijing at April 5-6.
or
on April 5 and 6.
or
from April 5 to 6.
which is acceptable to native speakers? And do you have any other suggestions?

Definitely not "at".
"On April 5 and 6" sounds as if you will visit on the 5th, then go home and then visit again on the 6th.
If you are leaving for the trip on the 5th, staying overnight and returning on the 6th, then use "from April 5th to 6th".