High level details about Okta Workflows can be found here.

https://www.okta.com/platform/workflows/

Here are the steps on how to map a profile attribute. You'll need to make sure you are pulling in the AD field "PwdLastSet" to your Okta user profiles.

https://help.okta.com/en-us/Content/Topics/users-groups-profiles/usgp-map-attributes.htm

Here are the Workflow steps I have that would be similar for your use case.

1. Create a parent Workflow with a Schedule trigger card and set it to run once a day
2. In the parent flow grab today's date and convert it to an ActiveDirectory date format (create and use child flow for this - see details at the bottom of this post to understand AD datetime format)
3. Call the "List Users With Search" Okta function card passing in a search criteria similar to "profile.ADaccountExpires lt adEncodedDate and profile.ADaccountExpires gt adEncodedDate24hrsPrior". Note that "adEncodedDate" and "adEncodedDate24hrsPrior" are values I derived from the previous encoding step. Select the steaming option and call a new child flow where you will either send an email for each record or log each to a Workflow table and then send one mass email later on.

Details about AD datetime format:

https://docs.microsoft.com/en-us/troubleshoot/windows-server/identity/convert-datetime-attributes-to-standard-format

"The Active Directory stores date/time values as the number of 100-nanosecond intervals that have elapsed since the 0 hour on January 1, 1601 until the date/time that is being stored. The time is always stored in Greenwich Mean Time (GMT) in the Active Directory."

@MynorR.50213 (Customer)​ if you get stuck with any of this let me know and I can ask if my management would be OK with me sharing a copy of our related Workflows.