With slackbot, is it possible to set a reminder for every last Wednesday of the month or last day of the month?
If you entered the following in Slack, you will be reminded on the 28th day of every month to [Complete My Task].
/remind me to [Complete My Task] on the 28th day of every month
I ended up using ifttt.com for this. You can do an "every last day of the month" trigger there and you can have it post to slack. It does require you to give ifttt.com access to your slack account and allow it to post on your behalf of course.