this is input
      date         category   sales
0     20/2/2014    weekdays  120.96
1     05/03/2015   weekend   120.96
2     19/05/2014   weekdays   75.99
3     09/02/2014   weekend    60.76
4     15/03/2015   weekdays   49.01
5     03/03/2014   weekend    50.3
6     09/05/2014   weekend    203.2

df2 = df.groupby(['date','category'], as_index=False)['sales'].sum()
print (df2)

date   category   sales
      2014   weekdays   196.95
      2014   weekend    314.26
      2015   weekdays   49.01
      2015   weekend   120.96