What is the penalty for missing a day of stoning?

Missing a day of stoning (Rami) at the الجمرات does not invalidate your الحج, but it does require compensation. The stoning on the Days of التشريق is considered واجب (واجب) عند جمهور العلماء, not a pillar (ركن), so missing it requires فدية but does not void the الحج.

If you miss the stoning on one day, you can make it up the following day. Perform the missed day's stoning first (all three pillars in sequence), then perform the current day's stoning. The stoning must be done in chronological order — you cannot stone for day 12 before completing day 11. All makeup stoning must be completed before sunset on the 13th of ذو الحجة (the last day of التشريق). If you departed منى early on the 12th, the deadline is sunset on the 12th.

If stoning is missed entirely and cannot be made up within the allowed time period, a فدية (sacrifice of a sheep in مكة) is required عند جمهور العلماء. The Hanafi school holds that missing even one day's stoning without excuse requires dam (sacrifice). If the stoning was missed due to a valid excuse (severe illness, disability), then appointing a proxy is the proper course of action rather than paying فدية after the fact. In all cases, the الحج remains valid.