Hi!
The format for date fields in the database is the format you guessed: yyyyMMdd. See this blogpost to see how you could check that out yourself in 2 minutes: http://developer.webnodes.com/developer-tip-luke
To do a Lucene query that finds nodes within a date range, you need to use the TermRangeQuery class from Lucene. Use it like you use a BooleanQuery with Webnodes (set the range query object to the custom query property on IndexQuery).
The constructor signature is pretty self explanatory (see below), but if you need a complete example, let me know, and I'll create it:
TermRangeQuery query = new TermRangeQuery("date", lowerDate, upperDate, includeLower, includeUpper);