Health insurance being dropped before last day of work

My horrible awful bosses informed me today they will only cover me until the week prior to my technical last day.

Basically I gave my employer 90 days notice for termination of our contract for last day to be December 15th and they got upset because I would no longer be here to cover their Christmas vacation. They tried gaslighting me into saying I had already agreed to cover it- which I had not. I ended up agreeing to try to help them out and working the 21st and 22nd for them even though it’s in the middle of my move across the country. Last week I discussed with them as said I was very overwhelmed with the move and would be living out of a suitcase on the 21-22 as my stuff would already be shipped. I asked if they would be willing to give me the 14th 15th off so I can relieve a little stress while still helping them on the 21st-22nd.

Well today my employer came to me to inform me they will only cover me with health insurance until the 15th since that was my technical last day. Is this legal? What can I do about this?