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 BG2 ToB 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