The seasonal weapons are only available for a limited period of time each year. The Egg-17 Launcher is only available during Easter time every spring, the Brain Washer is only available over the summer, and the Brain Harvester during Halloween.
To obtain the weapon, you need to make a purchase in the game, and it will be added to your inventory automatically. Please note that this offer is limited to 7 days after your first login to the game during the offer period.