I came across the expression "lilac times" for instance a Band's name Stephen Duffy and the Lilac Time or in a song line "[...gonna see the river man/ gonna tell him all I can/ about the plan/ for lilac time.]".
I really cannot think of a translation or meaning. What could it possibly mean? Got no clue.
Furthermore, is the expression "lilac time" a metaphor or an idiom.
But the context you are looking at is a music group called The Lilac Time, and who know why they choose that as a name. You could e-mail them and ask.
Yes - mailing them sounds like a good idea. The reference could be to an early 20th cent. poet, Alfred Noyes, but the context of his usage doesn't suggest anything figurative. He's talking about Kew Gardens - a botanical garden in the south-western suburbs of London: Come down to Kew in lilac time; when he says 'Kew' he's obviously referring to the garden rather than the suburb (or village, as it was when he was writing).
The lilac is syringa vulgaris, which flowers in late-spring/early-summer.