One Fine Day, a 1981 novel by Leon Arden, which can be borrowed (for free but registration required) from the Internet Archive. The author brought an unsuccessful copyright lawsuit against Columbia Pictures over the movie Groundhog Day. There is a very long and detailed description of the novel (and the movie) in the judge's opinion, which you can read at Film Suits.
Goodreads blurb:
Robinson Blake leads a cautious and unexceptional bachelor life in New York City. He has one minor vice (a secret [y]earning for his boss's wife) and two how to get up on time in the morning and how to end an affair with his eccentric secretary, Milly. Suddenly, Rob is caught up in a unique and terrifying calamity. A certain Monday becomes fixed in time and repeats itself over and over again, with Robinson Blake apparently the only person on earth who understands what's happening. Other people proceed through each re-run with no memory of what has passed before. Only Rob remembers. Rob's efforts to sort things right result in a series of comic and sometimes dangerous fiascoes. Then, surrendering to the erotic, forbidden possibilities of the situation, Rob sets about seducing the delectable Phillipa, his boss's wife. Every morning is a brand new day, and Phillipa must be wooed from scratch, though not always with the same degree of success. Finally Rob meets his nemesis, Mrs. Hawkmann, who is both the archetypical Jewish mother (Milly's) and a practicing, though not very competent, witch. Through Mrs. Hawkmann, who bears a striking resemblance to the late General de Gaulle, Rob is introduced to the Satanic forces he suspects may be responsible for this repeating day. But the final explanation of what is happening, as well as the solution to Rob's romantic entanglements, is as much of a surprise to the hero as it is to the reader.
The witch woman is not Gypsy but Jewish, the mother of the protagonist Rob's secretary and girlfriend Milly. In some iterations Rob hears on the TV news about the airliner brought down by an onboard bomb, in some iterations he prevents the disaster by tipping off the authorities with an anonymous phone call, and the last time around he boards the plane where he has a conversation with the Jewish witch:
Pushing past a man who had stood up to put away some hand luggage, I found my seat and knelt on it with one knee. Mrs. Hawkmann was staring out the window as if she hadn't moved since I left. My resolve not to mention our impending destruction took hold again. She turned and saw me.
"You look like death warmed over."
I could sense the metal, the air, our limbs, our lives at any moment, exploding, my wide eyes were already part of that imminent, loud, jolting detonation.
"What, already? What is it?"
"Bomb," I chittered. Gorna blowis up. Any minute. Bomb. On board. Here. With us. Really."
"You poor putz. I phoned them about that from my hotel before I left. It's true. I figured heartless you are but dumb you're not. To take a ride on a plane that's been crashing every day for a month, is dumb, am I right? OK you're dumb. I'm sorry. I thought you were smart. You're dumb. That's all, that's it."
In some iterations Milly kills herself:
At her downstairs door I am buzzed-in just as if nothing has happened and she is busy, as always, making that last-minute effort to finish dressing as I climb the stairs. In the doorway is the man who phoned me. Police Officer Dunne. He looks like a boy in a stolen uniform and seems to be assessing the exact extent of my sorrow. On the bed is the body covered with an unwrinkled sheet in which the symmetric folds from the Chinese laundry are still visible. A doctor, I am told, had come and gone. The young woman, he said, had killed herself with pills. She was discovered in her bathtub, the over-flowing water had leaked into the apartment below, which was how they had known something was wrong. I couldn't help feeling that Milly had gotten confused. People use a tub when cutting their wrists, a bed when taking pills. She probably got it all mixed up. Very like her to do that.
As far as I can tell from skimming, the weather seems to be the same every day, except for a catastrophic tsunami:
"It's a catastrophe unparalleled in world history," she recited like a child in school. The New York Post was held up like an eyetest. TIDAL WAVES STRIKE WEST COAST. FRANCE, THE BRITISH ISLES AND MEXICO ALSO HIT. MILLIONS BELIEVED DROWNED. OCEAN SHIFT IS WORLD WIDE.
The catastrophe was caused by "the world trying to correct itself," so to speak, as God explains to Rob:
"For example, the earth's spin of late has been slowing down. This was even detected not long ago by some French scientists. A fraction of a second was being lost each year. However, the deceleration would not have continued in terms of fractions of seconds. Beyond a certain point, the earth would have lost speed quickly and then completely. The result, a catastrophe. I had to step in just as I once did centuries ago. Yes, it's not the first time this has happened."
[. . .]
"Damn silly book. More trouble than it’s worth. The heavens and the earth were created in seven days, were they? Ha! Just this little adjustment today took almost a month. Anyway, the Bible does mention a time of great calamity and flood."
"Noah's ark," I snapped.
"A children's story. Charming. Yet the flood was real enough. It happened again, as you'll remember, just a few days ago."
"You mean those tidal waves all over the world?"
"I was a purist once. I tried to adjust the spin in mid-flight. Trial and error. Except one such error sent the oceans up onto the land. These days I take no chances."