This is equivalent to the Human Terran Birthday celebration.
The Archa celebrate the day they successfully crawled out of the egg sac, without being eaten by other sac-lings or their mother.
A healthy Archa female can lay over 2000 eggs and since their industrial revolution they do not have any natural enemies. Archa females (Sac Mothers) never let more than two (inn rare cases three) saclings survive.
Only very recently they are beginning to incorporate medical birth control methods.
Still the Hatchling or Sac Survival day is celebrated.
The usual celebration includes lots of ice cream (Since 5017 OTT ) simulated hachling crawl games and a memorial for all the saclings that didn't survive.