Bids are made on a yield basis. The rate shown is the average secondary market mid rate for 3-month bills, expressed using the standard euro money market yield convention (per cent per annum, using actual days to maturity and a year of 360 days).
Euro bills only have a single maturity date in any given month, namely the Thursday after the second Tuesday of the month. The end-quarter rate reflects the rate on the bill with just under 3 months remaining to maturity. The monthly average is for the bill maturing in the month three months later.