Dashalakshana Parva (Paryushan-Dash Lakshan) is observed by Digambar Jains after the main Paryushan Parva. It spans ten days and the Dashami (the tenth day) is the concluding day in the Digambar tradition. In 1988, the Dashalakshana Parva would have begun on the Shukla (bright) half of Bhadrapada and continued for ten days, with Dashami marking the completion of those ten days. For the exact calendar date of Dashami in 1988, you would need a 1988 Panchang or local temple calendar, as dates shift by location and calendar system. You can read more about the principle and timing of Dashalakshana Parva here and about the festival's significance here .