Thanks for the quick response.

Isn't setMonthSelection a proper setter for the variable? Because when i do public String monthSelection{get;set;} instead of the above, the variable is not set at all.

I tested some little things now and i realised that when i put the selectList in a second apex:form tag, everything is working correctly but that is only a workaround, therefor i would really like that button integrated in the pageBlock.

Here are the methods you asked for:

private Month month;

public Month getMonth() { return month; } 
    private void setMonth(Date d) { 
    month = new Month(d);  
    system.assert(month != null); 
  }

private void addMonth(Integer val) { 
    Date d = month.getFirstDate();
    d = d.addMonths(val);
    setMonth(d);
  }

And this is the class Month from which the object is generated:

public class Month {
	private List<Week> weeks; 
	public Date firstDate; // always the first of the month
	private Date upperLeft; 
	
	public List<Date> getValidDateRange() { 
		// return one date from the upper left, and one from the lower right
		List<Date> ret = new List<Date>();
		ret.add(upperLeft);
		ret.add(upperLeft.addDays(5*7) );
		return ret;
	}
	public String getMonthName() { 
		return DateTime.newInstance(firstDate.year(),firstdate.month(),firstdate.day()).format('MMMM');
	} 
	
	public String getYearName() { 
		return DateTime.newInstance(
		firstDate.year(),firstdate.month(),firstdate.day()).format('yyyy');
	} 
	
	public String[] getWeekdayNames() { 
		Date today = system.today().toStartOfWeek();
		DateTime dt = DateTime.newInstanceGmt(today.year(),today.month(),today.day());		
		list<String> ret = new list<String>();
		for(Integer i = 0; i < 7;i++) { 
			ret.add( dt.formatgmt('EEEE') );
			dt= dt.addDays(1);
		} 
		return ret;
	}
	
	public Date getfirstDate() { return firstDate; } 
	


	public Month( Date value ) {
		weeks = new List<Week>();
		firstDate = value.toStartOfMonth();
		upperLeft = firstDate.toStartOfWeek();
		Date tmp = upperLeft;
		for (Integer i = 0; i <= 4; i++) {
			Week w = new Week(i+1,tmp,value.month());	
			system.assert(w!=null); 
			this.weeks.add( w );
			tmp = tmp.addDays(7);
		}


	}
	
	public List<Week> getWeeks() { 
		system.assert(weeks!=null,'could not create weeks list');
		return this.weeks; 
	}

	/* 
	 * helper classes to define a month in terms of week and day
	 */
	public class Week {
	 public List<Day> days;
	 public Integer weekNumber; 
	 public Date startingDate; // the date that the first of this week is on
	 // so sunday of this week
	 
	 public List<Day> getDays() { return this.days; }
	 
	 public Week () { 
	 	days = new List<Day>(); 	
	 }
	 public Week(Integer value,Date sunday,Integer month) { 
	 	this();
	 	weekNumber = value;
	 	startingDate = sunday;
	 	Date tmp = startingDate;
	 	for (Integer i = 0; i < 7; i++) {
	 		Day d = new Day( tmp,month ); 
	 		tmp = tmp.addDays(1);
	 		d.dayOfWeek = i+1;  		
	 	//	system.debug(d);
	 		days.add(d);
	 	} 
	 	
	 }
	 public Integer getWeekNumber() { return this.weekNumber;}
	 public Date getStartingDate() { return this.startingDate;}
	 
	}
	
	public class Day {
		 
		public Date 		theDate;
		public Integer 		month, dayOfWeek;
		public String 		formatedDate; // for the formated time 	
		private String 		cssclass = 'calActive';
		 
		public Date 		getDate() { return theDate; }
		public Integer 		getDayOfMonth() { return theDate.day(); }
		public String 		getDayOfMonth2() { 
			if ( theDate.day() <= 9 ) 
				return '0'+theDate.day(); 
			return String.valueof( theDate.day()); 
		}
		public Integer getDayOfYear() { return theDate.dayOfYear(); }
		public String 		getFormatedDate() { return formatedDate; }
		public Integer 		getDayNumber() { return dayOfWeek; }
		public String 		getCSSName() { 	return cssclass; }
		
		public Day(Date value,Integer vmonth) { 
			theDate=value; month=vmonth; 		
			formatedDate = '12 21 08';// time range..
			//9:00 AM - 1:00 PM
			// three possible Inactive,Today,Active	 
			if ( theDate.daysBetween(System.today()) == 0 ) cssclass ='calToday';
			// define inactive, is the date in the month?
			if ( theDate.month() != month) cssclass = 'calInactive';
		}
			
	}
}