best time to buy ac and refrigerator,tv for cheap offline and online with card discounts ?

Online it's Big Billion Day on Flipkart which would be around the first week of OCT.

Offline it's definitely Diwali/Dussehra

P.S: ACs are usually cheaper in winters.

In General 

For Online
During Flipkart Big Billion Days (Usually in September or October) , Amazon Great Indian Festival (same time as Flipkart's Big Billion Days) , Reliance Digital Independence Day Sale (Around August 15) , Croma Festive Offers (Diwali and other major festivals) , Tata Cliq Luxury Sale (coincides with major sales seasons and festivals)

For Offline 

Includes the likes of Reliance , Croma , Vijay , Respective Appliances Stores (Sony, LG, Samsung  etc.) mostly during the period of Dussehra  & Diwali

For Air Conditioners:

End of Summer due to retailers try to clear out stock before the end of the cooling season
Festive Seasons Sales during festivals like Diwali and Dussehra

For Refrigerators

End of Financial Year  as retailers might offer discounts to clear out inventory before the new financial year
Festive Sales as Diwali, Holi, and Durga Puja, Independence Day , Republic  Day Sales

For TVs

Big Festivals such as Diwali , New Year with special offers and bundles
Year-End Clearance Sales towards December and early January

Card offers are mostly covered by HDFC , ICICI (2 prime banking offers ) for both Online as well Offline sales including EMI as well as Non EMI features. Also usually you get BAJAJ offers also in stores 
Apart from these other  bank cards offer time to time such discounts 

Additional Tip