When i went 11 o clock or 12
Between 9 am and 10 am week days.
Depends what week you go- anywhere from 10am to 2:30pm start time. There is a chart online that lists all start times (and firework displays) if you Google "opening times palace playland old orchard" and it's the first or second result that comes up.
I think they open around 9:00 am, maybe 8:00
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