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Open Sat-Sun-Mon from 10:30 AM to 4:00 PM.
Literal
Sat-Sun-Mon [genitive-の] AM 10:30 [from-から] PM 4 [until-まで] open.
A museum/gallery hours notice. 土日月 is an abbreviated list of days without the 曜日 suffix — standard in posted signs and schedules. The から~まで pattern ('from X to Y') is the Japanese equivalent of English 'X to Y' for time ranges and spatial extents. 開館 ('open [the gallery/museum]') is a compound verb-noun: 開 (open) + 館 (hall, building), used specifically for institutional openings (museums, libraries, halls). The sentence ends in the bare noun 開館 without a copula — typical sign/notice register where particles, copulas, and verbs are routinely dropped.