Halcyon Days EP was created in 2009.

Halcyon days are happy and meaningful days in the past that were full of peace and bliss.

The halcyon days of their youth were filled with endless sunshine and laughter.

"Halcyon days" denotes a period of time, not really a sense. So you could say, 'those halcyon days when i spoke to you'.

'Halcyon' as an adjective, means peaceful, calm, happy and carefree, after the halcyon, or kingfisher, which was believed to make its nest/lay its eggs out at sea.

You could perhaps say 'I felt a halcyon sense after talking to you.', but 'halcyon, other than in the set phrase 'halcyon days' sounds a bit pretentious.

Those halcyon days are still fresh in my mind.