I don't know about "huge" holiday but some do celebrate it. The first year I was married, my (very Reformed) ex-mother-in-law wished me a "Happy May Day". That sort of blew my mind, because I had always associated May Day with this...
Seasons Greetings and Happy Holidays. Tell them you wish them Happy Israel Independence Day or National Prayer Day this week.
May Day used to be associated with spring and maypoles. It was even a custom to leave "may baskets" on neighbors' doorsteps with flowers in them--lilies of the valley were traditional.