Nifty option Series

Can you say why there is no series for some months. Please check the expiry dropdown, no series for April and so on…

https://www.nseindia.com/live_market/dynaContent/live_watch/option_chain/optionKeys.jsp?symbolCode=-10006&symbol=NIFTY&symbol=NIFTY&instrument=-&date=-&segmentLink=17&symbolCount=2&segmentLink=17

at any point we will have 3 active series expiring at last thursday … you can trade on april series by monday
( and there are few long term option series as well )