What is the end-of-month option in Treasury bond futures, and why does it exist after the last trading day?

The end-of-month option arises from the structural gap between the last trading day and the last delivery day of a Treasury bond futures contract. Trading in the expiring contract ceases around the seventh-to-last business day of the month, but the short can still deliver through the last business day. During this window, the futures settlement price is frozen at the last trading day's close, while bond prices continue to fluctuate.\n\nTimeline Structure:\n\n`mermaid\gantt\n title T-Bond Futures Delivery Month Timeline\n dateFormat YYYY-MM-DD\n section Trading\n Active Trading :a1, 2026-06-01, 17d\n section Delivery\n First Delivery Day :milestone, 2026-06-01, 0d\n Last Trading Day :milestone, 2026-06-19, 0d\n End-of-Month Window :a2, 2026-06-20, 9d\n Last Delivery Day :milestone, 2026-06-30, 0d\n`\n\nHow the Short Exploits It:\n\nDuring the end-of-month window, the invoice price is locked (based on the final settlement price), but the short can buy bonds in the cash market at current prices. If bond prices decline during this window, the short profits by purchasing cheaper bonds and delivering against the higher locked-in invoice.\n\nWorked Example:\n\nFalconer Investments is short 50 T-bond futures. The final settlement price locks at 119-16 ($119.50). The CTD bond (CF = 0.8923) has a clean price of $106.75 on the last trading day.\n\nLocked invoice (clean): 119.50 x 0.8923 = $106.63\nDelivery cost on last trading day: $106.75\nNet: -$0.12 per $100 face (slight loss)\n\nThree days later, a weak employment report pushes yields up 8 bps. The CTD bond drops to $106.10.\n\nDelivery cost now: $106.10\nNet profit: 106.63 - 106.10 = $0.53 per $100 face\nOn 50 contracts ($5M face): $0.53 x 50,000 = $26,500\n\nOption Value Characteristics:\n\nThe end-of-month option is most valuable when:\n- The delivery month contains significant scheduled economic releases after the last trading day\n- Market volatility is elevated\n- The CTD bond has high duration (price sensitivity)\n\nTypically the end-of-month option is worth 0-3 ticks, though it can spike during turbulent periods. Like other delivery options, it slightly depresses the futures price below theoretical fair value.\n\nStudy delivery option pricing in our FRM course materials.