Although week numbers are never used in worldwide public holidays, it is possible to construct a recurring holiday rule based on an ISO week number, and this is sometimes used to specify the date of an insertion.
The fixed date recurrence rule for the weekday of a given week number is:
((7*(w-1))+(D-1)-3) days after the Thursday on or after January 1
where W is the week number and D is the weekday (running from 1-7 for Monday to Sunday).
For your convenience, the value of ((7*(w-1))+(D-1)-3) (the offset) has been calculated for the Monday (D=1) of all week numbers from 1 to 52.
Week |
Offset |
|
Week |
Offset |
|
Week |
Offset |
|
Week |
Offset |
1 |
-3* |
|
14 |
88 |
|
27 |
179 |
|
40 |
270 |
2 |
4 |
|
15 |
95 |
|
28 |
186 |
|
41 |
277 |
3 |
11 |
|
16 |
102 |
|
29 |
193 |
|
42 |
284 |
4 |
18 |
|
17 |
109 |
|
30 |
200 |
|
43 |
291 |
5 |
25 |
|
18 |
116 |
|
31 |
207 |
|
44 |
298 |
6 |
32 |
|
19 |
123 |
|
32 |
214 |
|
45 |
305 |
7 |
39 |
|
20 |
130 |
|
33 |
221 |
|
46 |
312 |
8 |
46 |
|
21 |
137 |
|
34 |
228 |
|
47 |
319 |
9 |
53 |
|
22 |
144 |
|
35 |
235 |
|
48 |
326 |
10 |
60 |
|
23 |
151 |
|
36 |
242 |
|
49 |
333 |
11 |
67 |
|
24 |
158 |
|
37 |
249 |
|
50 |
340 |
12 |
74 |
|
25 |
165 |
|
38 |
256 |
|
51 |
347 |
13 |
81 |
|
26 |
172 |
|
39 |
263 |
|
52 |
354 |
* note that if the offset value is negative, then the rule above should change from "days after" to "days before". For example, week 1 means 3 days before.
Week 53
The above table does not include week 53 (offset 361) as the above formula cannot be applied blindly as it requires the year to contain an ISO week 53.
For example, the formula correctly gives ISO week 53 beginning on Monday 28 December 2026. But, applied to 2027, the formula provides an ISO Week 53 date which is in fact that of ISO week 1 of 2028.
Topic 186610, last updated on 09-Nov-2026
