I think the best time to go on a day trip is any day except Saturday and Sunday when the ruins and cenotes and beaches will be full of local people. The weekend is the time to stay in Merida and enjoy a deserted centro historico.  The main market, Lucas de Galvez, is still open but it´s quieter in a way but more colorful in another as this is the day that some people will come in to town and shop wearing their traditional Sunday best.  Saturday I would shop.  Sunday the streets are free of traffic for the most part. A good day for a buggy ride in the afternoon.  In the morning there is the biciruta and the dancing in Santa Lucia Park.  The Zocalo is full of food and craft stalls all morning and early afternoon.  It´s a nice day to hang out.   Symphony concerts are Friday nights and Saturday afternoons at the Peon Contreras.  On Saturday night there is the Noche Mexicana in the park at the ´remate´de Montejo.  It´s fun.  The show can very from a folkloric dance concert from another state to a famous Mexican crooner or two.  Or it could be clowns and ballet.  It´s always fun and there are crafts to see and tacos and churros.  Ana Sabrina is even there and that is always worth the trip.