TransactionSearcher.cs

using System.Globalization;

public class TransactionSearcher {
    public List<Transaction> SearchByYear(DateTime date, List<Transaction> transactions) {

        return transactions.Where(t => t.Date.Year == date.Year).ToList();
     }

     public List<Transaction> SearchByMonth(DateTime date, List<Transaction> transactions) {

        return transactions.Where(t => t.Date.Year == date.Year && t.Date.Month == date.Month).ToList();
     }

     public List<Transaction> SearchByWeek(int year, int week, List<Transaction> transactions) {

        Calendar calendar = CultureInfo.InvariantCulture.Calendar;

        return transactions.Where(t => t.Date.Year == year && calendar.GetWeekOfYear(t.Date, CalendarWeekRule.FirstDay, DayOfWeek.Monday)  == week).ToList();
     }

     public List<Transaction> SearchByDay(DateTime date, List<Transaction> transactions) {

        return transactions.Where(t => t.Date.Year == date.Year && t.Date.Month == date.Month && t.Date.Day == date.Day).ToList();
     } 
}