Prime Minister Ariel Sharon's family and aides will solemnly mark his 78th birthday on Sunday at Hadassah-University Hospital in the southern Jerusalem neighborhood of Ein Karem. "It is a sad and difficult day," a Sharon adviser said. "The staff of the Prime Minister's Office will gather in the room next door to Sharon's at the hospital most of the day." Sharon has spent the past 54 days in the hospital after suffering a severe stroke. His condition remains serious but stable.