South African Mathematics Olympiad

Every multiple of $7$ represents a full week. Since today is Thursday, in one day's time it will be Friday. Thus each full week after today starts on a Friday and ends on a Thursday. $150$ days divided by $7$ equals $21$ full weeks with a remainder of $3$ days. The $147$th day from now will thus be a Thursday (end of $21$st full week), and consequently the $150$th day from now will be a Sunday.