Were Not The Right Man On Our Side
Old school Calvinists, such as myself, don't observe an ecclesiastical calendar and we don't celebrate any day as "holy" aside from the Lordsday. However, we do acknowledge special providences as occasions for thanksgiving. (Not a widely understood distinction).
That God used Luther's mailing (yes, that's an 'm') of the 95 theses to bring about eventual reform in His church is certainly a “former mercy and deliverance” for which I find occasion to give thanks.
If you are also thanking the Lord on this occasion, I think you will enjoy the entertaining Reformation film series and (if you can play mp3s) the educational audio series "Married To Another" (on the relation of law & gospel).