Earth-Hour’ was observed in the year 2015 on

Earth Hour 2015 fell on 28 March 2015, a Saturday. The campaign, run by WWF, asks households and businesses to switch off non-essential lights for one hour, from 8:30 to 9:30 pm local time. It is held on the last Saturday of March every year, so the date is predictable once you know the rule.

Why the other options fail:

• 31 March was a Tuesday in 2015, so it cannot be the last Saturday.
• 27 March was a Friday, outside the pattern.
• 19 March was a Thursday and far too early in the month.

The core concept is a symbolic, voluntary energy-saving campaign. It links to GS III (environment and climate change awareness) and to Mains answers on individual action versus policy-led mitigation. Earth Hour is a WWF initiative, not a UN or Government of India programme.

Trap: mixing it up with Earth Day (22 April) or World Environment Day (5 June), which are different observances on different dates.