Does anyone know how to make an item available only on certain days f the year?

For example:

if day = 25/12 then (register crafting recipe) else continue loading the rest of mod

To expand on Ben's explanation, you will always register the recipe. Whether or not the recipe is called depends on the day.

See here for how to get the month and day.

excuse me, but why not make your code like so;

    	Calendar c = Calendar.getInstance();
    	if(c.get(c.MONTH) == 12 && c.get(c.DAY_OF_MONTH) == 25){
    		//register recipe here
    		
    	}

 

excuse me, but why not make your code like so;

    	Calendar c = Calendar.getInstance();
    	if(c.get(c.MONTH) == 12 && c.get(c.DAY_OF_MONTH) == 25){
    		//register recipe here
    		
    	}

One mistake. Months start at 0, not 1. So check if the month is 11, not 12.

days too?

excuse me, but why not make your code like so;

    	Calendar c = Calendar.getInstance();
    	if(c.get(c.MONTH) == 12 && c.get(c.DAY_OF_MONTH) == 25){
    		//register recipe here
    		
    	}

 

Registration of recipes only happens during the loading process.  So it will only check on the day you start the server and not every day.

 

If this is just a single-player mod for you and your friends, then it would probably work okay though.  In other words, if someone using the mod will relaunch Minecraft every day your approach is acceptable.