Manchester United return to the Champions League after three years away, facing Azerbaijan outfit Sabah on… *checks notes* Thursday.A three-night Champions League matchweek? Pure gluttony. But that is the way of the competition in its modern format, an opening week designed to get maximum eyes on all 36 of Europe’s leading clubs.United fans may well feel transported back to their more recent Europa League endeavours when Michael Carrick’s side walk out the Old Trafford tunnel on Thursday. But fear not, the Champions League anthem will play, and they are back among Europe’s elite for the first time since the 2023-24 season.While Champions League football is synonymous with Tuesday and Wednesday evenings, the start of the season offers the rare quirk of all 18 opening league phase fixtures being equally distributed across three nights.And most notably for Premier League fans, there is plenty of opportunity to gauge each English club, with Liverpool and Arsenal the only teams playing concurrently on the opening week.The Athletic takes a look at why this is happening.Does this happen every year?Thursday Champions League nights have existed on the opening week of the competition since its expansion and the introduction of the league phase format in 2024.UEFA, European football’s governing body, said each of its three competitions would have one “exclusive matchweek”, meaning that where Champions League, Europa League and Conference League fixtures are usually all played in the same week, each competition would have one particular week to be showcased in all its own glory.The Champions League, though, is the only of the three competitions to receive the three-night treatment. The Europa League’s ‘exclusive’ week sees fixtures played on Wednesday and Thursday, while the Conference League’s bespoke week only sees Thursday fixtures.Come the second matchweek of Europa and Conference League fixtures, and all thereafter, fixtures will solely be played on Thursday evenings.Who else is playing Thursday?United welcome Champions League debutants Sabah to Old Trafford, kicking off at 8pm UK time.In another debut for the competition, Cesc Fabregas’ Como host RB Leipzig on Thursday, following a strong start to the season with seven points from three Serie A matches.Bayern Munich also play Thursday, hosting Bodo/Glimt, who last season knocked out Inter in a huge last-32 upset. Slavia Prague, meanwhile, face Lens.In Thursday’s early kick-offs, Fenerbahce welcome Roma to Istanbul, and PSV face Shakhtar Donetsk.Harry Kane’s Bayern Munich are also in action on Thursday. (Lars Baron/Getty Images)So when is the first week of Europa/Conference League?The Europa League commences with its first league phase fixtures next week, on Wednesday, September 16 and Thursday, September 17.Sunderland and Bournemouth will face AZ Alkmaar and Real Sociedad respectively, while last year’s Conference League winners Crystal Palace host Lech Poznan.With only six matchweeks in its league phase, the Conference League does not get underway until the prolonged September international break on October 15. This is another way in which it differs from the Champions League and Europa League, with its opening matchweek taking place on a typical Thursday which features all three competitions.The Conference League’s ‘exclusive’ matchweek falls on the sixth and final matchday of the league phase — December 17.Remind me how the new league phase works again?For the Champions League and Europa League, there are eight rounds of fixtures played, with the 36 teams contesting in a league table format.In a move away from the traditional group stages, the league phase is designed to be fair through a seeding system. This saw all 36 teams separated into four separate pots based on their ranking, with each team drawn two opponents from each pot. Four home fixtures each, four away.Crucially, in the league phase, teams cannot play against other sides from their same country, nor can they face more than two sides of one particular association.The 36-team table is separated into three sections: the top eight, who automatically qualify for the round of 16; ninth to 24th, with those 16 teams entering a playoff round to join the top eight in the knockouts; 25th to 36th, who are eliminated from the competition.The new format has sprung some surprises in its early life so far. Manchester City and Real Madrid’s struggles in the league phase during the 2024-25 season meant they both met in the last-32 playoff round, with City exiting in a 6-3 aggregate defeat.That year, Juventus were beaten by PSV in the playoff round, while Inter were the victims of an upset at that stage last year against Bodo/Glimt.