This file defines months/special days in year name and their length in days.
First column is just month/day number on the list.
2nd column is the duration in days.
3rd column is the strref to dialog.tlk to name of the month/day.
In BG1 TotS we have:

2DA V1.0
0
           DAYS       NAME       
0          30         15934
1          1          15935
2          30         16042
3          30         15936
4          30         15938
5          1          15942
6          30         15944
7          30         15945
8          30         15947
9          1          15948
10         30         15956
11         30         15957
12         1          15964
13         30         15967
14         30         15968
15         1          15978
16         30         15979