Examples of ultra-simple seasonal behavior differences

SpawnOnChristmas.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

// @kurtdekker - this is part of Jetpack Kurt
//
// Spawns different items during certain seasons, in this case Christmas.
//
// To use:
//	- place on empty active GameObject in your prefab or scene
//	- move GameObject where you want seasonal item to spawn
//	- supply alternate things to spawn (Christmas trees)
//	- (optionally) supply original things to destroy if we make a Christmas tree
//

namespace Seasonal
{
	public class SpawnOnChristmas : MonoBehaviour
	{
		// These dates could trivially be generalized for all seasons, but
		// I found it was better on the workflow to have different versions
		// for each seasonal event. YMMV.

		// from C# docs: The month component, expressed as a value between 1 and 12.
		const int FirstMonth = 12;
		const int LastMonth = 12;
		// from C# docs: The day component, expressed as a value between 1 and 31.
		const int FirstDay = 10;
		const int LastDay = 25;

		[Header( "What might we spawn on Christmas?")]
		public	GameObject[]	SpawnChoices;

		[Header( "Chance from 0.0 to 1.0 of doing it.")]
		public	float	SpawnChance;

		[Header( "If we do spawn, should we destroy something else to make room?")]
		public	GameObject[]	InstancesToDestroyWhenWeReplace;

		void Reset()
		{
			SpawnChance = 1.0f;
		}

		// conveniently other code can reach in here and change
		// other programmatic things like perhaps colors or music!
		public static bool IsChristmas()
		{
			System.DateTime now = System.DateTime.Now;

			if (now.Month >= FirstMonth && now.Month <= LastMonth)
			{
				if (now.Day >= FirstDay && now.Day <= LastDay)
				{
					return true;
				}
			}
			return false;
		}

		void Start ()
		{
			bool spawn = IsChristmas();

			if (Random.value > SpawnChance)
			{
				spawn = false;
			}

			if (spawn)
			{
				if (InstancesToDestroyWhenWeReplace != null)
				{
					foreach( var go in InstancesToDestroyWhenWeReplace)
					{
						Destroy(go);
					}
				}

				int which = Random.Range( 0, SpawnChoices.Length);

				Instantiate<GameObject>( SpawnChoices[which], transform);
			}
		}

		void OnDrawGizmos()
		{
			// programmer art Christmas tree... your tree might
			// be different size or shape, so hack up the numbers!

			float height = 4.0f;
			float skirtRadius = 1.0f;
			float trunkRadius = 0.2f;

			Gizmos.color = Color.red;
			Vector3 pos0 = transform.position;
			Vector3 pos1 = pos0 + Vector3.up * skirtRadius;
			Vector3 pos2 = pos0 + Vector3.up * height;
			Gizmos.DrawLine( pos1, pos2);
			Gizmos.DrawWireCube( pos0, new Vector3( trunkRadius, skirtRadius * 2, trunkRadius));
			Vector3 pos3prev = Vector3.zero;
			for (int i = 0; i <= 4; i++)
			{
				var rot = Quaternion.Euler( 0, 90 * i, 0);
				var pos3 = pos1 + rot * Vector3.right * skirtRadius;
				if (i > 0)
				{
					Gizmos.DrawLine( pos3, pos3prev);
					Gizmos.DrawLine( pos3, pos2);
				}
				pos3prev = pos3;
			}
		}
	}
}