on what day was it? | Did  you  mean  the day of week in  weekly weather table | Yes, in what say was the maximum low temperature in Amersham station.
SELECT t1.day_of_week FROM weekly_weather AS t1 JOIN station AS t2 ON t1.station_id   =   t2.id WHERE t2.network_name   =   "Amersham" order by low_temperature desc  limit 1
maximum low temperature refers to MAX(low_temperature); day of the week refers to day_of_week; in Amersham station refers to a JOIN condition where station.network_name = 'Amersham';