Sunday gap appears when the market opens on Monday with a price gap.
Sunday gap is also known as the Weekend gap.
Sunday gap develops when there is some political or financial breaking news on the weekend.
This Monday, EURUSD opened with a huge Sunday gap due to the French Elections results.
Elections and referendums are taken very seriously by the currency market.
Just after the Scottish Independence vote, GBPUSD developed a huge 300 pips gap.
On the day of Brexit voting, fortunately no gap developed but GBPUSD fell like a stone.
I have written this blog post on the recent EURUSD weekend gap.
Most of the time there will be warning before a gap opens.
Weekend gaps most of the time will appear in the direction of the trend.
Candlestick patterns are important warning signals before a gap appears.