Singer Jessica Simpson is keeping the date of her upcoming wedding secret from her close friends and family.

The star is rumoured to have picked America's Independence Day (04Jul14) for her wedding to Eric Johnson, and she fuelled speculation about her big day by throwing a bachelorette party in Los Angeles on Sunday (08Jun14).

However, Simpson's singer sister Ashlee insists even she does not know when the couple's wedding will be.

She tells Usmagazine.com, "I don't know when it is!... I'm really excited about it."

Ashlee will be a bridesmaid at the ceremony, and reveals she is pleased with the gown her sister has picked out for her.

She adds, "My dress is really beautiful! It's really, really stunning."