This observation came out from an investigation into force-merge performance impact on the cluster. It looks like weekly indices are force-merged every day of their week (offset by two days since this is what we tell curator to wait for). For example `ecs-default` for week 14 (considering a week starting on Mon that's 2025-03-31 to 2025-04-06)
```
2025-03-30 12:31:30,846 INFO opensearch log_request_success:258 POST http://127.0.0.1:9200/ecs-default-1-1.11.0-7-2025.12/_forcemerge?max_num_segments=1 [status:200 request:13573.687s]
2025-03-31 04:44:41,889 INFO opensearch log_request_success:258 POST http://127.0.0.1:9200/ecs-default-1-1.11.0-7-2025.12/_forcemerge?max_num_segments=1 [status:200 request:14466.217s]
2025-03-31 19:15:01,301 INFO opensearch log_request_success:258 POST http://127.0.0.1:9200/ecs-default-1-1.11.0-7-2025.13/_forcemerge?max_num_segments=1 [status:200 request:13420.837s]
2025-04-01 06:09:06,921 INFO opensearch log_request_success:258 POST http://127.0.0.1:9200/ecs-default-1-1.11.0-7-2025.12/_forcemerge?max_num_segments=1 [status:200 request:12950.229s]
2025-04-01 16:28:23,452 INFO opensearch log_request_success:258 POST http://127.0.0.1:9200/ecs-default-1-1.11.0-7-2025.13/_forcemerge?max_num_segments=1 [status:200 request:12906.148s]
2025-04-02 11:11:39,126 INFO opensearch log_request_success:258 POST http://127.0.0.1:9200/ecs-default-1-1.11.0-7-2025.14/_forcemerge?max_num_segments=1 [status:200 request:5213.472s]
2025-04-03 08:48:30,390 INFO opensearch log_request_success:258 POST http://127.0.0.1:9200/ecs-default-1-1.11.0-7-2025.14/_forcemerge?max_num_segments=1 [status:200 request:7380.601s]
2025-04-04 12:37:49,969 INFO opensearch log_request_success:258 POST http://127.0.0.1:9200/ecs-default-1-1.11.0-7-2025.14/_forcemerge?max_num_segments=1 [status:200 request:9460.734s]
2025-04-05 04:05:45,456 INFO opensearch log_request_success:258 POST http://127.0.0.1:9200/ecs-default-1-1.11.0-7-2025.14/_forcemerge?max_num_segments=1 [status:200 request:9917.410s]
2025-04-06 12:35:44,066 INFO opensearch log_request_success:258 POST http://127.0.0.1:9200/ecs-default-1-1.11.0-7-2025.14/_forcemerge?max_num_segments=1 [status:200 request:13213.116s]
2025-04-07 08:57:15,587 INFO opensearch log_request_success:258 POST http://127.0.0.1:9200/ecs-default-1-1.11.0-7-2025.14/_forcemerge?max_num_segments=1 [status:200 request:14749.986s]
2025-04-08 04:22:14,517 INFO opensearch log_request_success:258 POST http://127.0.0.1:9200/ecs-default-1-1.11.0-7-2025.14/_forcemerge?max_num_segments=1 [status:200 request:13124.516s]
2025-04-09 05:52:59,575 INFO opensearch log_request_success:258 POST http://127.0.0.1:9200/ecs-default-1-1.11.0-7-2025.15/_forcemerge?max_num_segments=1 [status:200 request:3990.150s]
```
As a side note, the same is true for yearly indices like `ecs-alerts` though those don't see many writes overall and force-merge takes in the order of 10-20 min