Responsa על בכורות 97:25

If a man heard about the death of his parent on the thirtieth day after such death, he must observe the seven days of strict mourning and the thirty days of mourning. But if he heard on the thirtieth day which was a Sabbath day or a holiday, he must observe only one day.
SOURCES: Pr. 436; Mord. M.K. 883; Hag. Maim. to Abel, 7, 1, 2.