In the context of UK takeovers, Day 21 is defined in the Takeover Code as the 21^st day following the date on which the initial offer document is published. An offer must initially be open for acceptance until the later of Day 21 and the date on which the offer becomes or is declared unconditional or lapses. Day 21 is the earliest date on which a bidder could lapse its offer on the acceptance condition, provided it has published an acceptance condition invocation notice at least 14 days before the relevant date.

For further information, see Practice note, The offer timetable: Day 21: Minimum period for which offer must remain open for acceptance; earliest date on which bidder could lapse offer on acceptance condition if it has published an ACIN.