The worst week on record for shares in Spirit Airlines Inc. took a turn late Friday, when the cut-rate carrier and its would-be merger partner, JetBlue Airways Corp., appealed a regulatory decision to block a tie-up of the airlines.

Spirit shares jumped as much as 17% in late trading Friday after news of the court filing landed, adding to a 17% advance in the cash session. Overall, the stock plunged 55% in the holiday-shortened week, the worst showing in its 13-year trading history. Skepticism was mounting over Spirit’s ability to survive as a stand-alone company after a federal judge ruled JetBlue’s $3.8 billion takeover would harm cost-conscious travelers who rely on Spirit’s cheap fares.