def number_of_tweets_per_day(df):
    """ 
    Function will count number of tweets per day

    Arguments:  
            takes a Pandas DataFrame as input

    Returns:    
            a new DataFrame grouped by day 
            (new column 'Date') with the 
            number of tweets for that day
    """

    df["Date"] = df["Date"].apply(lambda x: x.split(" ")[0])
    # performs the groupby and count actions and saves to return
    last_df = df[['Tweets', 'Date']].groupby(['Date']).count()

    return last_df